[MunchkinMan]

I'm not sure what other methods of adding Monsters to the combat you might be considering (there are a few, but to my knowledge none of them act incredibly differently than by use of Wandering Monster), but in every single instance I know exists, I can say the following: Any time you are facing multiple Monsters, you add the combat strengths of all the Monsters and then the Munchkin side must beat that combined number. There is no complicated formula to figuring it all out, fortunately. Complicated formulas would probably drive lots of players away.