[Radek]

First of all, Kangaxx the Demilich is a "half-undead". He should be undead but some anti-undead tools, like False Dawn do not work against him. Turning Undead need not work against him, too. I don't know.
Second, Kangaxx the Demilich is a level 27 creature. Your turning abilities depends on the level of the undead to be turned. It is not correct that your level should be higher than the level of your victim but, in my experience, your level must be at least near the level of your victim. Therefore, you should be level 25 or so if you want to turn Kangaxx.
Note: Kangaxx the Lich is also level 27. If you suceed to turn Kangaxx the Lich then you can think of turning Kangaxx the Demilich. Again, you can fail even if you should be successful. Turning undead need not work against the Demilich.