[Barry the Sprout]

I'm going with Six on this - these guys shouldn't actually give you any trouble at that level. Just use huge quantities of healing potions and magically nuke them to bits. Abi Dalzims is obviously quite a handy spell for this kind of time. Any insect spells from Jaheira work wonders as well. Then just send in the two rangers in your party buffed up to the hilt. In theory you should be picking bits of giant off of the ceiling for weeks...

If its the later bits you're having problems with I can only advise resting and repeating. Or getting someone to use the Wave halberd - it instantly destroys Fire-elementals and Burning Men. That can speed up the whole process of causing carnage...