Hoo boy! I wish you and your party much luck, that’s a really fun encounter.

I was the DM for that, my group: Tabaxi ranger, Yuan-ti necromancer, Human battlemaster. When the great Lich appeared, our wizard cast fly on the fighter, who used the disarm maneuver on Acererak, took his staff and broke it over his knee, causing a massive AOE that he barely survived. This was enough to deal sufficient damage to make Acererak retreat while giving him another reason to hold a grudge. It was very epic and I was proud of my party for using such great tactics.