Mephisto usually likes making fun deals in our world, at the expense of fucking with someone to the highest degree.
My first guess would be that he'd be immune, like Captain America is. Is he limited to what Cas can do? Or did he bring along all of his powers with it?
Okay. Sooo people who open up dimensions. Anyone that can create portals? Open rifts? A way to get him out of Cas's body without killing anyone (or I'll throw Logan your way, he's a great scapegoat) and get him locked away and in another dimension. Ideally.