It’s a lot better balanced if you activate /players 8 as a console command. It used to be available in singleplayer and LAN characters (those were the same thing), but since they got rid of LAN characters, it’s only available in Single Player. But you level up a lot faster, and IIRC you hit Duriel around level 24 or 25, maybe even higher than that. So you have a lot more options on skills you can use against Duriel.