If you're playing the 5.0 or the 5.1 then the Boss Lich is on the first floor but there's slimes in the way so yeah.
I'm having this same problem actually on version 5.1. When you enter the Lich cave, it automatically starts you on the highest floor you last visited instead of Floor I, so I can't go back to try the Lich Pool event.
EDIT: You can reset the dungeon with one of the ladder escape items you get from entering it, it will always reset at 1 so you don't have to redo anything to get the Lich event to work.