Wait, you haven't played the D:OS campaign yet? Or are you referring to older games?
The level-up boss sounds pretty hard to balance to me. Even a single level up is going to make the fight nearly impossible to win if the boss is any strong at first. Unless he starts a couple levels below you. Then you'd have three or four rounds to kill the idol. Or if killing the idol made the boss drop a level or two (or even reverse the leveling process entirely). Lots of opportunities for character text or even midcombat dialogs (if that's possible).
