Running into Baal for the first time. Anyone who's played a summoner before will know that his Decrepify curse is absolutely the worst. It probably decreases my DPS by 50%.
This wave took me 5 minutes, but gave me a ton of XP even at level 42.
Finally time to kill the bastard. Just for this battle, I put a point into the skill "Desecrate", which spawns corpses to be used. In the end, Baal didn't kill many of my skeletons but the battle did take plenty of time.