[Phantom Lord]

You'll need a +4 weapon to hit Kangaxx in Demilich form. The only exception is the Mace of disruption, since Kangaxx counts as undead it won't hurt him physically but still can kill him outright.

As far as I know, the only shops that sell +4 weapons are Adventurers Mart (Staff of Rhynn) and - given ToB is installed - the Vigil Knights in Watchers Keep (Short Sword of Mask). Of course there are other weapons in the game that are powerful enough, but these have to be found first.

Kangaxx takes only a small amount of damage damage per hit, so the fight may take longer than berserk mode lasts, on the other hand I think that Kangaxx casts only 1 Demilich Howl (save or be dead) and 9 imprisonments (see the protections listed by Bruce). Btw, Kangaxx never changes targets, so once he targets a protected person, the fight is practically over.