I've read the manual but don't fully get multi-monster fights. It says you can't chose to fight one, and flee from others. But do you sum up all the monster points and try to beat that in one single combat, or do you fight one monster at a time?
The monsters combine their strengths: you don't fight them one at a time.
This was confirmed by the "Munchkin Czar" at the Steve Jackson Games Forums, and who would dare to argue with a Munchkin Czar? (You may find extra clarification of how multi-monster fighting works by reading through the thread, if you're still at all confused.)