Alright my prank starts off with a port a potty and a fire hose.
The fire hose would be attached to the port a potty through the small vent in the top of the toilet.
As the victim does there "#2" the fire hose would be turned on causing the highly pressurized water to be sent down the vent into the poo tank. This in result will hopefully send shit and victim flying.
Sort of like a shit tsunami.