Yeah wait a few levels. He keeps casting stoneskin so make sure you bring down those. Strangely pierce magic seems to but not breach, use the wands of spell striking. I ignored the demons except for the glabrezu who I killed because they were easy. Mass area effect spells work against most of his minions still, so it's worth using them. You seem to only need +2 weapons to hurt him too, so missile fire is good. I did lots of damage with arrows and bolts. Even with four fighter types I still went through a lot of healing potions and several wand of resurrection charges. Sometimes he summons less monsters than others so that can make it easier.
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