Can I play Go Up a Level cards on another player – for instance, to make him go up to a level so that a monster that would previously ignore him will now chase him?

This is not the original intent of Go Up a Level cards (GUALs), but it is such a munchkinly and vile idea that we liked it too much to say no. But just as when playing a GUAL on yourself, the munchkin must legally be able to gain that level. You cannot play a GUAL card, even one with a secondary effect, on a munchkin at Level 9, and you cannot force another player to do something he doesn't want to do (e.g., discard an Item) by playing a GUAL on him.