The only time I fought them I had way more XP than the default for it (PC Fighter/Thief, Sola, Tashia, Kelsey, Keldorn and Jaheira about 6 million XP each); managed on the first try without pre-buffing. In fact, having no buffs lead me to the key to success.
When I tought "well, since I have nothing up..." my first action was a dispel magic from Keldorn right in the middle of them, followed by Sola dimension dooring to the casters, PC activating assassination, Kelsey and Tashia casting improved alacrity and Jaheira activating Greater whirlwind.
Keldorn's dispel removed some of their annoying buffs, and their sorcerers dragon's breath activated Kelsey's and Tashia's Chain Contingencies at 50% Health (3x Abu Dalzim at nearest enemy each) interrupting their castings of improved alacrity : (
Jaheira, Keldorn and PC gang up on their fighters one at a time while Sola hacks away with Tsuki no Ken and Flail of Ages at the casters, his innate MR protecting him from the nastiest attacks. At the first Total Eclipse two of the figthers and the Sorcerer were dead, one of the fighters injured. During the Total Eclipse, Sola runs up and finishes the lone fighter. Tashia and Kelsey drink some tea totally forgotten at a corner safe from harm's way.
Keldorn's dispels, both from innates and from carsomyr were the key