Originally Posted by
The Giant
Turn Undead specifically says right on the card that you can't use it against Xykon. Treat Xykon as simply not being there for the purpose of using that shtick; the card just ignores him. Note that you CAN use it at Range 0 even if Xykon is the top monster, it just doesn't affect him. Technically, the top monster IS undead, even if it is a monster with a specific immunity to the shtick, making the conditions needed to use the card true. Thus, you can use the shtick.
Turning Xykon's undead out from under him is a great strategy, by the way. You bypass Xykon's ultrahigh Defense because he cannot be affected by the card!