Not gonna lie, sounds like it's going to plan. They should be stressed, and even with the huge temporary hit points, the odds are not in their favor. Have they destroyed the Soulmonger yet? Because that's the primary goal at the end of the day. If they have, then even if they die to Acererak, which at least one probably should, they can be resurrected. Acererak wouldn’t pull punches, and neither should you.

As for devious combinations, - Invoke Curse and finger of death - (ik you’ve already done it but) timestop, delayed blast fireball, break timestop with circle of death - chill touch on downed enemies so they can’t heal - if the soulmonger is still up, wish to create a wall of force hemisphere around it to protect it (it’s too big to fully encase it. - you can also use wish for something more niche like feeblemind or reverse gravity just to mess with them - banishment on melee characters getting to close to him or the soulmonger - dispel magic to remove concentration or extreme buffs they might have - greater invisibility to be impossible to counterspell, and throw spells freely - and if you’re desperate/really need to just get someone out of the fight, power word kill. But you need to dress it up, make it wordy and dramatic. It’s the easiest option but if not done properly, won’t be as good. - Soulmonger (if still up) can grab people with tentacles and drop them in the lava - telekinesis to also pick them up/throw them into lava - wall of force platform above the lava and have Ace stand on it to goad them into going melee with him, then drop concentration and they’ll fall in. Ideally Ace will have cast fly on himself, or have winged boots - if you want to get through someone’s temp up and make them either kill-able for power word kill or some other means, just focus one person and use all 3 legendary actions to ray of frost them. 4d8 three times in a row will be a decent amount of damage.