Since we currently have the skills applied after the dice roll for all skills, and what with exceptions being a bad idea, here is my fix:
After a player has piled on roll a d6, if the result is less than the strength of the player he is piling on then the piling on player is stunned.
Probably need rewording but you get the idea:
Black Orc piles on a Human Catcher, rolls a d6, on a 1 Orc is stunned.
