Ok, this guy is not easy, and there are very few ways of killing him, the easiest I have found is, SPOILER
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First kill his lich form, then, as he is transforming, cast mgic immunity, Abjuration (I think) on one of your characters who has a +4 or more weapon, then just lay into him with that character and he will eventually go down.
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Nicodemus Deathslayer, Paladin of the Order of the Holy Flame