Thoughts on Acecerak in the context of this thread:
He's quite clearly supposed to fight the party solo.
He's got twice the hp and twice as many spell slots as a regular Lich, which is good, but won't be enough.
Against medium-level characters (or unoptimized high-level ones), obviously drop the huge bonuses conferred by the Nine Gods.
If you don't want to, or the heroes are well-built high-level ones, my only suggestion would be to have Acecerak break all rules and go all-out.
* give him [1 + number of heroes] Legendary Actions each round for starters. This gives him the minimum number of legendary actions needed to steer the Sphere against one character, and still do something after each other hero's turn.
* enable his lair actions and let him take each one once in every round: at initiative counts 20, 10 and 0.
* Acecerak has Int 27 - he's able to concentrate on as many Concentration spells as he likes. Since he's alone, he will potentially have to make up to a dozen concentration saves each round, some of which will be DC 20 or even DC 30.
* so he needs three Legendary Concentration saves apart from his three regular Legendary saves - he can't afford to spend his regular Legendaries on maintaining concentration (a notorious gap in the defense of legendary ceatures is how worthless concentration spells are for them)
* talking about Legendary Saves, let him regenerate one Legendary Save each round
* make the staff sentient so it gets its own initiative count, on which it itself makes Acecerak take the Invoke Curse action (the power is cool, but if this combat is going to become interesting, Acecerak needs to take the Cast Spell each round -
every other action available needs to be a bonus action or some other extra action or it will simply never get used)
* when
he casts Time Stop, he doesn't cast the puny 5E version. When he casts Time Stop, he casts the real spell, the one he learnt from previous editions!

Basically, he ignores all restrictions. Meaning in round one, he casts 2 or 3 lethal attack spells (Banish, Maze, etc) at the party
and a nice buff spell or battlefield control spell. Then in round two, he does that again. This helps immensely in action economy parity. This should nicely mean that at least one hero is disabled during each of these two rounds, balancing the fight immensely (as the others must decide between re-enabling their friends and attacking the lich). Of course, if they suffer three or six bad rolls in a row, Acecerak will probably TPK them, but shouldn't that be a real possibility - after all it's Acecerak we're talking about!
* give him 500 temporary hit points, so he will at least survive their initial onslaught. Otherwise the risk is he dies before he even gets to show off all his cool powers. Remember Strahd? Players quickly found out that his temp hp buffer was pathetically small, and that was against single-digit level heroes. This is the thread for high-level PCs! So lets give Acecerak
ten times that amount - believe me, he's going to need every single point.