The Antithesis Theory.
If Slendy came into being (or in Robert's POV, came over from the Other Side) due to our thoughts, can we pull over something to fight him?
He could be small-fry compared to some of the things over there.
If he's a purely Thoughtform being, well, we do have other thoughts you know. A Hero? Like Zero's hope, but in the form of not placing the title on someone, but making someone/thing to fit the title?
I need to think of a way to test this.