I'm very interested in this question, I'll be running ToA soon so the way you present it makes it sound like the final boss is pretty trivial. I think the a correct answer would be to shed light on the fact that the dungeon itself also poses serious risks for the party, which can affect them later on.

After looking through the book I had one specific risk to mention:

The

Atropal

must be defeated

before Acererak is summoned.

This creature is far from a piece of cake, especially considering on of its legendary actions

Wail: (Costs 3 Actions). The atropal lets out a withering wail. Any creature within 120 feet of the atropal that can hear the wail must succeed on a DC 19 Constitution saving throw or gain 1 level of exhaustion

Although this is costly to use, I believe the primary point of this action is to make the party more susceptible to the final challenge. It is also worthwhile to note that this foe is highly naturally protected by the final area it is encountered. Battle map link, big spoilers

Considering the area, the fact that this foe will

Hover throughout the entirety of the fight, making it hard for melee characters to output a lot of damage.

And that this foe will be particularly tanky because it

Has magical resistance, is immune to two damage types and many conditions, and a negative energy aura that directly cancels out party members buffed by the trickster god (if they are within 30 feet).

Not to mention the fact that it also has a

Life Drain attack which has a range of 120 feet with a DC19 Constitution saving throw

This foe will no doubt stay alive for a couple of rounds to get in some devastating

Wails

Also, consider the

Summon Wraith action. The wraith itself has a CR of 5, a life drain attack and can raise a party member that goes down to fight against them as a Specter. If the Atropal manages to get two wraiths onto the board, this encounter can tip very quickly against the players.

After getting through all that, only then can the players face the final challenge, and I don't think they'd all come out without

A couple of levels of exhaustion. If any character gets to at least the 3rd level of exhaustion, they'd be truly in trouble for the final fight.

It's intentional.

The difficult climactic battle is the one before the final battle. If you did the work, the last battle is supposed to be a cakewalk. If you didn't do the work, you can easily get murdered. But since your players did the work, they get the payoff of having the last battle be a fun little grandstanding moment.

The alternative would be having the final battle be totally unwinnable if you didn't gather the spirits.

If you really want to, you could halve the effect the spirits have, but even then it's a really huge benefit that makes the final battle all but a foregone conclusion.

By the way, just as a side note:

I'm not sure if you were saying this or not, but gaining temporary hit points isn't the same as healing and wouldn't be altered by curses or exhaustion.

The final encounter was trivial for our party. Here are some tweaks to consider.

We ran the final encounter. It did turn out to be both trivial for the final encounter but felt non-trivial for the players along the way. Here is what we mean.

One strategy that made the combat easier for the PCs was this:

PCs hid in the Sewn Sisters layer - then ran down the stairs for ranged attacks - then used their movement to go back up the stairs.

This strategy caused the following problems:

The tentacles on the Soulmonger could never reach the PCs because they were always more than 30’ away. The tentacles couldn’t reach the stairs. Additionally, the PCs targeted the struts which only had 100 hit points. As a result, they were never in danger from the SoulMonger and no PC ever got pulled into the lava.

Additionally, because of that range:

The Atropal’s necrotic aura was also ineffective. The aura only has an effect if the PCs start their turn within 30' of it. Because the PCs were running into and out of the room, they essentially never started a turn in range of it.

The actions of the initial opponent that did cause some growing concern in the party were these:

Wail: The Atropal used it Wail effect every round. Before the combat was over, every PC had 2-3 levels of exhaustion. This was highly concerning to the party - but resulted in very little effect in combat because the SoulMonger's tentacles never reached them and there were otherwise no Ability checks they needed to make.

The result of this setup was that when the final opponent arrived, the party was fully healed, and though they had on average two levels of exhaustion, the final opponent was dispatched trivially:

Acererak appeared, lost initiative to most of the party and took heavy ranged damage in the first round. His first spell, Time Stop was countered by the two mages in the party. (The second mage countered his Counter spell.) Acererak was damaged by the fighter and hurled into the lava. Ranged attacks killed him before he even had another chance to cast a spell.

Here were a few details we observed that could make the combat less trivial for the PCs and perhaps a bit more dramatic without making it deadly:

• Ready Actions: If the PCs are hiding in the Sewn Sister's room, the Atropal could spend a lot of time floating around with no opponents in site. Have the Atropal use a Ready Action for its Life Drain - so the moment a PC runs out to attack with a ranged shot - it can attack them. Also stay out of melee range near the ceiling or floating over the lava.

• Use Longer Soulmonger Tentacles: To give the SoulMonger any chance to defend itself, extend the length of the tentacles to 45' so it can reach into the stairwell. That way if PCs run down into the stairwell and shoot the struts, the Soulmonger can still try to grapple them and pull them into the lava.

• Counterspells: When Acererak appears, if there are two mages in the party, be sure to use a higher level spell slot for Counterspell - otherwise a simple Counterspell of a low level Counterspell can shut down a high level spell.

• Acererak Strategy: Here's a spell strategy for Acererak. TimeStop is cast which gives him time to cast Shield, a Wall of Force sphere to isolate a group of PCs, followed by Power Word Kill on any mage that would counter his spells. Of note, in our game Acererak was killed by the party before he got a single spell off.

That said, it might not matter to try to balance the final battle - balancing the pre-battle may be all that is necessary. As noted in one answer above, making the final battle "trivial" might have been intentional by design. The result might be good for story reasons. For this game, the players were ecstatic and laughing that it went so quick in spite of their terror. The encounter seemed to feel epic for them. One player even asked to save the chat transcript because they thought it was so cinematic.

So you may be in good shape as a DM either way.

I just got done DM'ing ToA with some friends, here's how they dealt with this encounter...

First of all the party consisted of:

1. 12th Level Cleric (Unkh)
2. 12th level Wizard (Papazotl)
3. 12th level Paladin/Warlock (Kubazan)
4. 12th level Sorcerer/Warlock (I'jin)

They all had a spirit helping them as I showed above.

They did not have a ridiculous amount of magic items with them, mostly what is found in the adventure.

They arrived and decided to deal with the Atropal first. They killed it in two rounds. It managed to use its summon wraith ability, however that wraith got turned by the cleric (even with advantage on the saving throw). It also used its yell/scream twice which gave one player 2 levels of exhaustion and another player 1 level, the other two passed both saves.

So basically they nuked down the Atropal, turned the wraith, and then Acererak shows up...

As it says in book all the players gain 50 temporary hit points if they are inhabited by a spirit (they all were).

Acererak rolled low on initiative so some of the players got to go first.

The Sorcerer/Warlock Used Hex and Eldritch Blast, then twinned it, so thats 6 Eldritch Blast rolls...some of them hit some of them missed. The Paladin/Warlock misty stepped to one of the balconies to get closer to Acererak and shot eldritch blast at him. The Wizard I believe tried to cast a 5th or 6th level spell (I cant remember which one) but Acererak counterspelled it. On the ends of these turns I used Acererak's legendary actions to move the sphere of annihilation into people and use the invoke curse from the staff.

Then it was Acererak's first turn, it states in the book that he will use all his power to slay the characters, but it also leads you to believe that Acererak will underestimate his enemies because of how powerful he is, so going off this I decided to have him simply cast Finger of Death at the incoming paladin that would surely be trying to get into melee with his pesky smites and staff of striking (found off the goatman earlier in the tomb), the paladin failed but the 50 temporary hit points left him not too bad off. (I kind of roleplayed this as Acererak realizing that the spirits were helping the characters).

Now its the clerics turn, here's where things got interesting. Who would guess that this arena plus a second level spell would trivialize the fight? As it turns out, Silence has a 120 ft range but counterspell only has a 60 ft range, even if Acererak still had his reaction to cast counterspell he could not have counterspelled the Cleric's Silence because the Cleric made sure to cast from about 70 ft away anyway. The other important thing to note is that literally every spell on Acererak's spell list requires the verbal component. Lastly, Acererak enters the room on that balcony by the mist gate, so he is literally stuck there unable to cast or move out of the silence zone.

Rolls back around to the Sorcerer/Warlock who again, Eldritch Blast + Hex attacks Acererak and twins it, 6 more eldritch blasts. The Paladin/Warlock Misty steps from the balcony with the phylacteries to the mist gate balcony and attacks Acererak twice. He used his highest level spell slots to get 4d8 radiant damage, he also used charges from the staff to add 2d6, and because of the spirit he did 3d6 pschyic plus the die from the weapon, and of course 2 attacks so it ends up being a total of like 8d8 and 10 or 12d6, I can't remember exactly. Regardless, it was a lot of damage.

The Wizard I believe then cast Sphere of Invulnerability this turn hoping to protect himself.

Because of Silence the only thing Acererak could do with his legendary actions is to move the sphere and use the paralyzing touch on the paladin (which failed)

Acererak's 2nd turn...he was already below 100 hit points at this point I believe, however because of Silence he could not teleport away so I decided to keep going. If you use the spell list that is in the book for Acererak, the only way he can get off the balcony is to jump and then cast Teleport mid air (Why the F does he not have misty step?) So I had Acererak teleport on top of the Soulmonger, now this sucks because it took an action, (misty step would have been a bonus action).

As you can imagine, the Sorcerer/Warlock again eldritch blasts him and twins it, however this time he reacted with shield to block more of the hits.

The paladin on the balcony was kind of stuck because of the silence zone so he missed a turn.

The Wizard I believe cast Ice Storm on Acererak which was almost negligible but still did some damage, however this hit the Soulmonger, the tentacle then reacted and hit the wizard and threw him in the lava, however the 50 temporary hit points and the Abjurer's shield blocked all of this fire damage.

Acererak could have cast teleport to leave at this point but I instead had him cast time stop and then Circle of Death, they all failed their saves and took ~40 damage but the guy who got cursed took 80, however the 50 temporary plus the fact that they hadn't been injured much meant everyone was alive.

By the time it rolled back around to Acererak he was killed by the cleric ending silence and the paladin and warlock shooting him with more eldritch blasts

The wizard took 55 fire damage but also gained 50 temporary then misty stepped out of the lava to safety and used a spell to finish off Acererak. (its exactly 30 ft down from the edge of the balcony to the lava)

The cleric almost died from the wraith getting a critical hit when it broke out which was funny but they killed it and then simply stood 30 ft away from one of the struts and shot it til it broke and won.

So in summary: Your player's will have a REALLY hard time if they do not cast Silence or if they do not have a lot of Counterspells, also if they can't get into melee range or if they do not have the spirits helping them. If they attack the Soulmonger right away and get thrown into the lava and don't kill the Atropal quickly they could also run into trouble.

I'm about to run the final encounter tomorrow! My group has been playing in ToA since last March, so let's just say, the build up to this final fight is at its peak.

Something I haven't noticed in any online discussions is Acererak's use of minions, which is surprising. There are alcoves in the Cradle of the Death God specifically for summoning skeletons to fight the party. I plan on incorporating at least 12 skeletons and like 6-8 cultists of varying difficulty into this fight. My players also had the eye and hand of Vecna from an earlier encounter, which Acererak stole from them in the jungle on the way to Omu.

Suffice to say, the final fight is going to be much harder than the book anticipates. I rolled the stats for the two Vecna artifacts and have added them to the Atropal's stat block, making him a very formidable opponent. I'm using a modified stat block for Acererak as well, giving him access to misty step and a few other fun spells. My players are level 15, so I needed to give them something of a challenge for this fight. I don't think I'm going too crazy with the modifications, but I'll try to let people know after the game tomorrow. :)

In addition the answers provided here, I think it's important to note one thing that wasn't mentioned, but plainly worded into the adventure:

In addition to the power granted by his or her trickster god, the character gains the following benefits while he or she can see Acererak

This includes the temporary hit points gained by the trickers gods

As written in Tomb of Annihilation,

Acererak

does not have the spell

Greater Invisibility

prepared, but giving that opponent that spell will make them immensely more powerful. It's really a question of when you would want them to cast it since

It can also prevent counterspell, and Acererak is not likely to have his concentration broken easily, so concentration spells won't be going anywhere soon.

Combine that with

Silence, which has no saving throw,

and you will find this opponent suddenly devastating.

Combine the last suggestion with

Swapping out power word, kill with psychic scream (which requires no verbal component) and is 9th level

and someone will surely die. It should be easy to modulate when that first spell is cast which should turn the tide if it becomes too easy. Simply use the opponents flaw:

"I underestimate the resolve of my enemies."

Modify his spell list, let him prepare, milk Power Word: Kill

Even under AL rules, we can in change the spells Acererak has prepared¹. This is sorely needed, because frankly, his spell list sucks. It is full of redundant damage dealing spells most of which he will never get to cast, while it sorely lacks any spells that give him defense, mobility and control.

Survival times

A high damage character (not hyper-optimized with Sharpshooter or Great Weapon Master) on level 11 will deal near 30 damage per round, with the extra 3d6 psychic from the trickster gods about 40. That means in one round, a party of 4 characters can be expected to deal about 160 damage to Acererak (it could be much higher, with a lucky crit on sneak attack or smite, for example). He has 285 hits. He won't last two full rounds.

So, winning initiative, and trying to remove a character as quickly as possible is important (see below on how to do that). If he goes first and gets rid of one character, this drops to 120 damage, leaving him with 165 after a round. If he again gets rid of a character in round two, it drops to 80 damage and that would leave him under 100 afterwards, at which point he would teleport away.

Technically, the AL rules also allow you to increase the hits of a creature to the maximum of their HD, in his case this would be 390 from 30d8+150, which would also buy him resilience for another round with an extra ~100 hp, but for an individual creature like him that seems like cheating to me. I think this is meant for something like orcs, where there could be variability in hp from one orc to the next.

From this, and the fact that a normal fight only is expected to last 3 or so rounds, it is clear that he only will get 3 actions, and never cast most of the level 4+ spells on his list during the fight (he can cast level 1-3 with legendary actions). He should use these actions to force through his highest-level, highest-impact spells.

Spell list changes

You can modify the spell list in ways do not change his AC, or hp, or damage output per round:

• Spell Level 1: false life instead of the sucky ray of sickness upcast to level 8, this would add another ~40 temporary HP to his hp, thereby technically not changing his hp, and allowing him to fight for a third expected round. I probably wouldn't do this one as it is not in the spirit of not increasing hp, but might do it for a game outside AL. Mirror image might be a better option, it stays for 1 minute. He can cast it for free with a legendary action to refresh it.
• Spell Level 2: include spider climb on level 2 instead of knock. If you want to give him a use for his Bonus action, misty step instead of arcane lock. He can cast it as a legendary action, to get out of a wall of force prison without needing to disintegrate with his action.
• Spell level 4: instead of the sucky 4th level damage spells, give him dimension door, fire shield, and improved invisibility. Being invisible will make him mostly immune to counterspells, and negates the trickster god effects. (Note that he has Truesight 120' so this same trick will not work against him)
• Spell level 6/7 Consider replacing one of chain lightning, circle of death or finger of death with freezing sphere or delayed blast fireball - they have the advantage that you can cast them "for free" in a way if you are in time stop.

Enter the fight prepared

At the very least, you should assume he has mind blank going, as that has a 24h duration, and he should have this up every day all the time, even if he is immune to most conditions like charmed or frightened. It also will negate the psychic damage the trickster gods would add, sparing him 40-50 damage or more each round.

Ideally, he should cast some other buff spells upon himself before facing the characters. The adventure only mentions that he appears if the characters kill the Atropal, and doesn't say if or how he prepares. It doesn't sound like there would be a lot of time between the death of the Atropal and his arrival for vengeance. Still, he must realize that whoever was able to destroy the Atropal must be a somewhat dangerous opponent and he should take precautions. For example, he could pre-cast fire shield on himself, to soften up characters that damage him (costs no concentration and lasts 10 minutes).

You may of course play up his "he underestimates the characters" trait instead and play him as arrogantly going in unprepared, believing in both his superiority and relying on his ability to reform. In that case the fight will be a pushover. And that may be OK, too, but here you are asking about how to make the fight non-trivial, within the rules of engagement.

Going first

Acererak has a Initiative bonus of +3 from Dex, not great, and no match for characters that have maxed Dex and initiative boosting features like Alert (+10). What spells can he pre-cast to improve this? There is not a lot unfortunately. The only wizard spell published that improves Initiative is Gift of Alacrity, a subclass specific spell from Adventurer's Guide to Wildemont adding a d8. The only core spell is enhance ability,Cat's Grace option, which will at least give him advantage on the Initiative check as that is a Dex check for an effective +5. It will require his concentration, but he can drop it afterwards.

Dealing with silence

A highly effective tactic against liches is casting silence on the area they are in, so they cannot cast any spells. We killed multiple liches that way in our last campaign. Acererak has an Int of 27 and hundreds of years of experience, he should know that how important it is to avoid silence. There two main things he can try, and ideally he'd be prepared to do both:

1. Stop silence from being cast
2. Have a way to move out of silence

A direct way to stop silence is to counterspell it. For this reason, you might be careful to not cast shield against an mundane attack until the casters have been taken out, so he can use his Reaction to counterspell.

Getting out of silence

The issue is that once there is silence, he cannot cast any spells with a verbal component to get out - which is basically all of them. He has no natural climb or fly speed and no equipment for it either. The tiny balcony he appears on will not allow him to do move out without plunging into lava. He needs to fix that.

The best way for this is a contingency spell with a trigger that needs no sound. If you allow this in your campaign a contingency dimension door can teleport him out. If you do not allow dimension door, spider climb of fly would work but require concentration. He also could have either of those pre-cast, but then could not boost initiative with enhance ability. (Contingency doesn't need to be on the spell list, as it would have been cast in the past).

As a last measure, he might also be able to steer the sphere of annihilation to create a ledge on which he can walk out of the silence area.

Power Word Kill

The other approach is of course to take out any character that looks like they could cast silence, i.e. cleric, bard, ranger, maybe warlock.

Temporary hit points do not change the character's hit points. If the character has less than 100 hit points, then Power Word Kill just kills them, no save, no nothing.

The adventure is for characters level 1-11. If you assume characters are level 11 at the final encounter, then they would have well below 100 hit points. For example, a cleric that can cast silence with a d8 hit die and +2 from Con would have 8 + 10x5 (taking the sure hits which statistically is better than rolling over many rolls) + 22 hp, or 80 hp. A wizard able to cast counterspell would have 6 + 10x4 + 22 (if they have high enough Con), or 68. Even if they use upcast aid, they wouldn't clear the 100 hit point ceiling at that level.

The characters can try and counterspell the PWK. This will not auto-work, as PWK is a level 9 spell, and counterspell only works reliably if it is of at least the same level as the spell it tries to counter, and an 11th level caster can at best cast it at 6th level. So the player's counterspell needs to overcome a DC of 19 (10 + the target spell's level) with a spellcasting ability check. On 11th level with maximized spellcasting attribute of +5 on the check, that has a 35% chance, as 14 to 20 will succeed to meet the DC.

He also can in turn counter the counterspell, and as actions are his prime resource to consider, he should use a slot high enough the players cannot match it, that is 7th level or up. A second counterspell counterspelling his counterspell for a level 7 cast has a 45% chance to succeed (he won't be able to recounter once more as he only has one Reaction). Both PC counters need to succeed to get through, so overall the players only have about 16% to stop the PWK, if they have two characters that can counterspell, else someone is toast for sure. .

With his second action, he can repeat this process to take out a second character.

With his third action, he could send off someone who he thinks to be of moderate Intelligence into a no-safe maze spell, from which he can be expected to not return before multiple rounds have passed. However, as outlined above, if all characters attack at this stage he likely will be low enough on hits to teleport away.

Time stop

To stay in tune with the narrative of him wanting to dish out punishment, he might first kill one character with PWK, then after eating a ton of damage in return, cast time stop to fortify himself a bit, especially if you do not let him prepare himself. He can use this to cast spells unless they affect

a creature other than you or an object being worn or carried by someone other than you

He can for example, do 1-5 of:

• upcast false life on himself
• cast mirror image on himself
• cast greater invisibility on himself (supposedly these do not stack)
• cast fire shield on himself
• cast wall of force to encapsulate one or two adventurers and remove them from the fight (ideally clerics or fighters less likely to be able to teleport out)
• cast animate dead, to animate who he just killed
• cast freezing sphere as a bead, or delayed blast fireball, to overload the refreshing trickster god temp hp when combined with another damage spell

I think preparation and straight PWK is the stronger approach, but this one might be more fun and flashy for showing off to the players.

Breaking the staff

Lastly, his Staff of the Forgotten One has this action

A creature in possession of the staff can use an action to break it over one knee or a solid surface. The staff is destroyed and releases its remaining magic in an explosion that expands to fill a 30-foot-radius sphere centered on it. Each creature in the area must make a DC 18 Dexterity saving throw, taking 132 (24d10) force damage on a failed save, or half as much damage on a successful one.

If he is really pissed off, and sees he'll not win the fight, combining this with his 3-action Disrupt Life legendary action on the turn before his own would total up to 174 damage to all around him, without any chance to refresh temp hp. Even after the 50 temporary hp, this has a good chance to kill several characters and allow him to go out with a "boom". He will reform, after all.

¹You are playing under the Adventurer's League guidelines, which state under "Adventure Adjustments":

Creature Adjustments. When adjusting an encounter’s difficulty, you may not create new creatures, change the creature’s number of actions, or affect Challenge Ratings. You may [...] change hit points within the range afforded by their hit dice, and carefully follow non-Challenge-Rating affecting DMG “Minor Alterations” and “Traits” guidance.

The guidance on modifiying creatures in the DMG (p. 279, for the 2014 rules under which this question is) says

Innate Spellcasting and Spellcasting. The impact that the Innate Spellcasting and Spellcasting special traits have on a monster's challenge rating depends on the spells that the monster can cast. Spells that deal more damage than the monster's normal attack routine and spells that increase the monster's AC or hit points need to be accounted for when determining the monster's final challenge rating

So, as long as they do not dead more damage than the spells he already has (e.g. no meteor swarm on 9th), and they do not increase his AC or hp, you can modify the spell list.

I am currently running ToA and my characters are in the Tomb of the Nine Gods. So for the Atropal fight, I am going to have the hag sisters involved heavily (as our cleric's mission is to rid them and free one of the children's souls for a noble in Baldurs Gate).

The final fight, I found a homebrew version of Acererak that is more befitting the lich that gods want dead, but can't find his phylactory, nor deal with him...it kind of makes him OP, even with the spirits help, so I plan on holding nothing back. Just like the lich wouldn't. Which means that most likely a good portion of the party might get taken out, but with the soul monger gone, that wont be a problem.

I changed a lot in the campaign, but to keep it relatively shorter, I added primordial pieces or stones that have special magic properties, playing up the whole amazing background of Chult. The primordials play a huge part in Acereraks plan...as written I felt like acererak didn't have a clear intent for his mission, I just made him operating in Chult because the regular gods cant interfere with Chult in any way because of the deal struck in the war of the primordials. Ubtao betrays his fellow Promordials, he gets this land without interference. Only catch is protecting Dendar.

Enter Acererak who is using these souls to siphon into what I dubbed the unborn god, a god who was killed before the primordial war. With this god born in Chult, he/she can intervene in Chult, which means unleashing Dendar and the Primordials and waging war on the gods.

However....gods do what they want, and was already born again. He's currently with the party masquerading as Orvex. This stops any chance of Acererak TPKing the party, while letting them feel the unrelenting wrath of the lich. The god...I havent 100 percent decided what he is going to do, but he has been floating the cosmos for a millennium...so he wants to experience the world, (spoiler he may end up becoming someone they need to fight end endgame).

This allows them to stop the death curse, interrupt Acereraks plan (kind of), technically he succeeded...but they get victory with dozens of possibilities for future campaigns.