Stoneskin and Adolquium stack. You can stoneskin before a fight to help reduce damage taken from the boss' first AoE. On bosses with adds, where you know the tank will have trouble grabbing them immediately, Stoneskin + Adolquiuim prior to the add phase on yourself will probably be the difference between a wipe and a successful run. At least, that's how it has been for me in the lower dungeons.