You might need to level up a bit more if you're having problems. There's not really a technique to fighting him, you probably just need more experience, more HP and better weapons.
Take advantage of your greater numbers. Use summoned creatures no matter how weak, put them in between you and him. Use Haste. Make your thief invisible beforehand and backstab him. Attack him from behind. Use missile weapons as the only weapon he has is a melee weapon.
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Memnoch - Custodian of the Order of the Holy Flame 