[Nerull]

He's tough but doable. Helps to have spell resistance and (especially) high saving throws. Throw caution to the wind, and use all of your buffing/healing potions that you have. Dispatch his minions as quickly as possible, then get into melee range with the big guy. The two times I beat him I played a cleric (easiest of all, just cast Spell Resistance, then turned away all of his minions, then went into melee with him), and a fighter (tougher, hacked through his minions as quickly as possible then closed into melee). With an arcane spellcaster or druid, your best bet is to use your area effects to dispatch the minions as quickly as possible, then keep a summon/henchman in melee against him at all times, taking down his defenses as much as possible.

Oh, and stay spread out (have the henchman/summons attack nearest or stand your ground, then move well away from them). This will stop him from using area effect spells as much.