Really depends if the Xeno can damage the metal on Jason X's body, which seemed to be tough as hell(bullet-proof, at the very least.) I'd *think* it would be able to, given that Xenos can shred through most alloys that comprise Predator tech/armor. /shrug
And if that is the case, the Xeno should win. Granted, the Xeno can't kill Jason, but it could definitely incapacitate him long enough to constitute a forum win-- especially if it decapitates him(his neck is not armored, after all.)