Summon a lot, DON'T CAST TIME STOP. Try sending your summons far away from the party, so that you can bash him with ranged weapons while using your summons as cannon fodder. I find that having one person (in your case prolly Sarevok) bash on him as much as possible, have him shift (via cloak of sewers (LOVE THAT ITEM
)) into a troll, regenerate and then repeat. Also works with regenerative items. I like using weapons more than spells against him, but you should take a magical approach (after all, seems like you have a little
bit of magic power.)