When I try to kill Kangaxx the lich (not Kangaxx the demi lich) he goes down to almost dead, but then he won't die. I keep on beating him and he is damaged (I have dealt him like 10000 damage or so) but he refuses to die.

I have killed his lich form before, but now it doesn't seem to work anymore.

Did you use the fake talk attack? I think you need to have him go on his spiel before he'll die.