If they're doing it right (angling their shield towards the ledge and slightly down) then they will be knocked off the stage regardless of which side you hit their shield on.
Actually, they should jump out and shine instead of bair if they see you up B early. Cause after that you can't recover
This doesn't work if they do it right (floatier characters should be fast falling after you hit their shield. This way they'll be able to beat you to the ledge every time.)
It really sucks when people actually practice the Marth killer and do it right every time. Makes it much harder to get around <_<