I myself am not strictly convinced that Slenderman walks around here in the human world, or killing people.
But it exists as a symbol. A terrible symbol, created by and scarred into the popular consciousness. The symbol exists; it is as real as Pepsi or Television or Democracy. And it hurts us. The pain and fear it causes is real. That's how symbols work.
And that's why the symbol works against Slenderman.
I have taken it, as I mentioned, raw and screaming from the pages of Fell, where it has been charged by intense energies from tens or maybe hundreds of thousands of people reading it. Intense feelings of wonder, of hope, of horror and of disgust has been directed at it. This gives us a headstart in making it a significant symbol against Slenderman.
You're thinking, how does that work exactly? How can I transfer the immaterial but tremendous energy of the symbol just like that? The answer is: By deciding to do that. I will probably suffer some psychic backlash from handling this massive accumulated force, but I have done it. It's not hard to handle the immaterial powers of the imagination. They are after all immaterial.
Slenderman was born the same way, so it seems the obvious route by which to fight it.