Go Up a Level played in combat?

[Eccid]

I played Munchkin Booty at a friends house the other day (awesome game btw) and we ran into a situation that i wasn't sure about. One of the players was level 8 and when he opened the door it was a level one monster with no bonuses, he instantly played a "Go Up A Level" card and then beat the monster to win. I said that I didn't think he could, but the rules didn't specifically say you couldn't use that card in battle, so the owner ruled that he could do that to win. Can you use "Go Up A Level" cards in battle? If so, I'm completely changing my munchkin strategy :P