KANGAXX SPOILERS
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I was taking on Kangaxx (in lich form), and he just wouldn't die. He stayed at near death for a looooong time. So I tried control+y, and it wouldn't work on him. I'm fully patched, and this is the first time I've encountered this. What's up with it?

Were you using a protection from Undead scroll?

I've found often that if you have one of those active, that the undead peeps can't go to their death text and become their next incarnation, ala the vampires in Bodhi's lair and Kangaxx. It's happened to me once or twice.

If not a PfU scroll then perhaps a bug preventing him from speaking.. you might need to check your files out.

Did you give him a 'chance' to die? It could be like a troll... Something similar once happened to me when fighting Kangaxx in lich form.. I reloaded and it worked fine.

Maybe I didn't give him a chance to die each time, I don't know. However, I wanted to just have the battle done quickly, so I used a PFMagic scroll.
I closed the game, reopened it, and was able to kill him this time. I'm fully patched, so it's wierd that he won't die one time, but will die after reopening the game...