Welcome to Kangaxx the Demi-lich, one of the toughest battles in the game
Well, it would be anyway, if beating the guy was not already so well documented. If you trawl the archives you'll find a lot of stuff written about Kangaxx (entitled 50 ways to kill a demi-lich). I can't remember them all off hand (I'm sure someone will reply with them)
for the record he's magic immune, needs to be hit by +4 weapons and casts imprison at a rate of knots. The way I dealt with him was a scroll of magic immunity (wasteful) and the improved mace of disruption. Most anti-dead weaponry and spells will have a good effect, but I liked the scroll as the imprison is blocked out by the shield it creates. There are cheaper and more efficient ways to finish him though...
hope this helps