Hmmm slendermane drives everypony insane with his stare. Fluttershy stare will make anything run with fear making her insane only causes said thing to fear more. How long will it take for slendermane to die of fear? How long will it take for Fluttershy to die of insanity? Answer both and you know who will win. Oh even if slendermane can't fear Fluttershy's stare will make him fear.