It's not as hard as it looks, trust me. You're going to want to do this one in a local game, for a very good reason. Local games allow you to try again if you fail any stage of the level; there are three stages in Mountain Pass. If you fail in a multiplayer game you have to start from the first stage.
Whether you're doing it by yourself, or with three friends you know, you're going to want to pick your characters very carefully. I went with Cartman, Craig, Pip, and Jimmy.
Whenever you get a special with any of the characters, make sure you wait until there are a ton of enemies on screen before you deploy, that way you can make the most of it. Always place the computer players in spots where they are always throwing snowballs, to gain their special while you're not controlling them.
- Stage 1
- Stage 2
- Stage 3
Once you're at Wave 8/8 on Stage 3, MANBEARPIG!
He walks super slow and has a lot of health, but he is so so easy. In the middle where it loops under the rocks is where you are going to trick the MANBEARPIG.
When he reaches the middle, you're going to want to open up the left side of the map. After, you're going to want to close off the right side exit of the rock loop, so he starts walking the other way (make sure you ALWAYS HAVE AN EXIT FOR THE MANBEARPIG before you close a section off). Once he gets to the left side, do the same thing, block the left side and open the right side. He will be trapped in here until you kill him.