Summon creatures. (If you have ToB, go to the first level of the Watcher's Keep and get the Summoning Golem Book, really useful - go to the crypt and get the spider figurine - get a wand etc..)
Buff your party and the creatures. (haste and AC at least)
Set traps around him whil his circle is still blue.
Make space between your party members so, as stated above, only one at time gets hit by aera effect stuff.
Engage him with summoned creatures only. His circle turns red, he takes some damage from traps.
Makes your char start firing projectiles at him (arrows+2, arrows of ice, bullets+2 etc...)
(You can engage with one char who has the sword of dragon slaying that you should find in the same dungeon, but at least 1 round after you sent the summoned creatures)
Cast greater malison on him. Breach. Then some magic missiles/skull traps/cone of ice (? - don't know the name in english [img]smile.gif[/img] ).
And when your last summoned creature dies, cross your fingers, drink potions, and with some improvisation, imagination and a tiny bit of luck, you should kill him [img]smile.gif[/img]
Good luck.
