[Morgeruat]

with Jaheira you can cast insect swarm/plague it's great for keeping him from gettting spells off, at this level hitting him shouldn't be a problem, and make sure to let summons take the brunt of the assault letting your fighters hit him from the sides, Keldorn should use his dispel magic ability, then his true seeing, the dispel will take out many of his spell protections, minsc should hit him hard and fast, Valygar should haste himself (and anyone nearby) and lay the smack down on him. Imoen should use the spells mentioned above, (lower resistance, pierce magic, greater malison, and then proceed to keep using breach, and similar spells to take out his stoneskins, and other combat protection spells).

mountain bears, or fire elementals from Jaheira are great for cannon fodder, as are spiders from Imoen, area affect spells like Abi-Dalzim's Horrid Wilting are perfect as they do massive damage, and don't hurt the friendly units, skeleton warriors from Imoen should come in after the bears and elementals start dying off, and basically just keep imoen and Jaheira back, using wave tactics so you can get your characters back to heal from J, while Imoen keeps summons ready to go into the fight.

hope this helps