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 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.