Step 1 The Supplies
You're going to need a lot of poop and pee for this one. Get guys with sling shots and squirt guns. If it's possible get a fire hose to shoot ALOT of piss at someone.
Step 2 Pick the victim
Doesn't necessarily have to be a cast member, but someone associated with the jackass movie.
Step 3 Attack
Have one person who is in on it to bring the cast member to a place where everyone is hiding and he can't escape. Once he's trapped in this certain location, either alley way or whatever, everybody comes out and gets him all at once with people sling shotting poop and squirting him with piss with squirt guns and/or the fire hose. Could you imagine spraying someone with gallons and gallons of piss? Especially piss that would actually hurt.