OK, 6 level 16 PCs is probably to much for a solo Acererak, even with my simple suggestions that buff him to CR 26. What you need is something in the quick fix #2:All characters are level 16 and will be when they face Acererak and the atropal.
Okay, so the major damage dealer is the tiefling swashbuckler rogue who recklessly throws himself into danger. Since the characters had previously gone through Curse of Strahd, he's a wereraven. (He volunteered to be resistant to nonmagical damage instead of fully immune.) Thankfully, all of the attacks will either be magical or of a type to which he's not resistant.
Next up is the halfling barbarian/tempest domain cleric of Kord. He has a magic hammer that is essentially Mjolnir, and one of his go-to spells is shatter.
Then there's the elven rogue/cleric/ranger whose favored enemy is the undead. He has a magic bow, the archery fighting style, an extra attack, and the sharpshooter feat, so he can deal a ton of damage.
The wild card is the eight year old girl with a fiendish patron. She likes to set things on fire and uses the bonfire cantrip anytime she can. She also has the use of counterspell, so that can usually screw up the villain's plans for a couple rounds.
The half-elf sorcerer/fighter also has counterspell, plus she has an extra attack. She also has a +3 staff, but I'm hoping most of the party will choose to avoid close quarters combat.
The one who can't avoid close quarters combat, however, is the dwarven fighter battle master. Honestly, he doesn't do nearly as much damage as one would think a high level fighter would do. He's pretty tanky, though, and has an AC of 21.
So yeah, six PCs with a lot of actions per round. That's why I worry that the enemies will just get wailed on and only occasionally be able to dish out some significant damage.
- Use the CR 25 version (replace raise dead with fireball / lightning bolt)
- Make it Mythic: give it the Mythic Trait: when he hits 0 all conditions and effects on him end, he regains all of his hit points, and he can use mythic actions (if you want to add them). Mythic actions don't change the CR, just give you more options. The XP of Mythic CR 25 Acererak is equivalent to a CR 30 monster is about right for 6 lvl 16 PCs