[Barry the Sprout]

My money goes on lowering resistance and battering with spells. Use summons to keep the pesky critter busy and then blast him with lots of lightning and magic missiles. With enough spell casters this works very easily. Also be prepared to use stuff like wands and once a day items. Horn of Berserker summoning, figurine of Kitthix, etc are all useful for when he kills or wing blasts your other summonses. And when he is almost dead send in the tanks - it speeds things up and they have little risk of dying by then.

Good luck and happy hunting!