Kangaxx casts 1 imprisonment spell per round, and sadly no - it cannot run out so get blasting and kill the thing.
The easy way is to buy a protection from magic scroll from the adventurer's mart and go and smack him.
Oh, by the way - you need a +4 or +5 weapon to hit him, I think
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Greetings mortals, my name is Wah, Arch Lich Wah.