Don't know, I just beat it up usually
Well, you do need +3 or better weapons I think. Those should be available rather easily in the game. Protection from Fear will help, though I have beated it without those as well. Free Action might help, don't know, never used it as it dies really easily..
Basically give +3 weapons to the tanks and then whack it. Get Melf's Minute Meteors to the caster and pelt it away. It's not that tough at all, when you have the gear..