I just finished Kangaxx recently (only one reload thanks to the trick about using the wizard eye as the focus of the demilich's imprisonment spells) but I'm confused because I thought Kangaxx could only be damaged by +4 weapons.
What happened was I ran Minsc up to the demilich dual wielding Crom Faeyr and the MoD +2 while the wizard eye received the brunt of the lich's spells. After 2 lower magic resistance spells and a whopping 13 damage from Minsc, Kangaxx just keeled over and died. I got several messages - critical hit, target immune to damage, target save vs death - which I assumed were successful blows from the mace. I went a mere three rounds before Kangaxx failed a save, curled up and died.