My personal view, I stand on the side of:
Yes, make it so that he won't follow you off the level.
No, to buffing him/his level any further. He's a badass. A badass with weaknesses. And those weaknesses can be exploited, in the same way that any other badass with weaknesses in the game can be. I don't feel that the XP gain (which is different depending on how many cats you've killed throughout the game) is particularly a great and horrid thing. As many people have said: he's deadly as it is. If you're ready for him, you can beat him fairly well. If you're *not* ready for him, he's going to eat you alive. And having been the recipient of that 'eaten alive' at least once, I have great respect for his power. For me, doing the quest isn't just about getting an awesome ring. It's about keeping a very angry, very powerful creature from kicking my ass. :)