Some more tips to dispose of the Don
Usually the chest in his room has the trap of "Guardian" on it, set it off intentionally and the guardian will attack the Don, let it wear him down for you as well. Also potions of razor cloak help, and everyone should carry potions of restoration to deal with any damage/poison he deals out. Usually if I am going to kill him I take out Calzone before I meet the Don so I only have to deal with one at a time as well. Also he is generally easier to hit with missile weapons than with melee ones. Also the best way to get his items is to PP him dry before you kill him