Shedninja is definaitly stoppable, it has the most weaknesses out of like every type combination, and that long list of things that own it. I'd say Gengar or Rotom. I'd say Rotom becuase Rotom-H is the best counter/check for Scizor, can trick a scarf onto walls like Blissey and still hurt things.
Ghost/Bug is weak to Rock, Fire, Dark, Flying, and Ghost. That is 5.
Grass by itself is weak to Fire, Flying, Ice, Bug, and Poison. Still 5.
Rock by itself is weak to Fighting, Steel, Ground, Water, and Grass. 5 here too.
Bug/Flying is weak to Rock, Fire, Electric, Ice, and Flying. Same thing.
Rock/Water is weak to Grass, Fighting, Ground, Electric, but I'd say that is actually worse, due to the sheer usage of Earthquake, Close Combat, and even Grass Knot. But still, that's 4.
Ghost/Ice is weak to Rock, Ghost, Dark, Fire, and Steel. Yet another 5.
Ice/Ground is weak to Steel, Fire, Grass, Water, and Fighting..
Ice/Psychic is weak to Dark, Steel, Fire, Ghost, Bug, and Rock. That is 6! Wow.
Grass/Psychic is weak to Fire, Dark, Flying, Ice, Poison, Ghost and Bug. 7. Sucks to be you, Celebi.
Grass/Ice is weak to Fire, Flying, Poison, Bug, Fighting, Rock, and Steel.
(Not that that should matter anyways, because Obamasnow vs Shedinja = Hail KO unless it's a BP Shedinja.)
Bah, I'm tired of writing, but there are other, worse combos out there than Shedinja's, so silence :P