Yep, perfectly legal.
Pay attention to the ranges on the Death Magic spells tough, they are fairly short so that Slaughtermaster must get pretty close, and he has to do hits to take your levels away. Spirit Leach doesn't work as it doesn't do any hits. He's got two spells that will work (if he gets both) and one is only 12" so make sure you know which ones to stop.
Also, use Cogitation, it can easily make him lose concentration on his first attempt.