Last time I did it I sent Korgan in there alone, butt naked and wielding only a toothpick. No, not really, I was joking.
I used his enrage and greater whirlwind and just had him beat the thing to death with a rune hammer.
If you don't have Korgan, cast spell immunity abj or use a protection from magic scroll on your best fighter, send them in alone, and whirlwind attack with a +5 weapon.

Well if you have a Sorcerer in your party just use Dragon's Breath on him and he will be toasted. :D