Anything to kill Shedinja before the opponent even has an opportunity to attack? See, that Shedinja can't be least powerful, because at least it makes the opponent use 1 PP and takes a turn off effects like Trick Room and weather. What you really want is something designed that it would make as close to no difference if it wasn't even there. That's the absolutely least powerful.
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"As the size of an explosion increases, the number of social situations it is incapable of solving approaches zero." -Vaarsuvius, Order of the Stick