[Barlik Longbeard]

This works all the time, so long as you have high enough wizards and preists. Stay in a corner of the area, then cast cloud kill on him a few times (just to wound him to make it easier to kill him). Then cast low level summoning spells and send the creatures in at a different angle to you one at a time. This uses up the dragons spells and nasty stuff. Once he just attacks normally summon your best creature (generally the deamons work the best, and don't forget to use the usual protection from evil 10' radius before). Then send them in and then your warriors. Hack and slash your way till he is dead.

Alternatively you could just lower his magic resistance with some spells and then cast death spells at him till he dies. Nuf said.