Let Him In: The Secret To Beating The Fiend
There's a sign on the door of the Funhouse that reads "Abandon all hope ye who exit here." An obvious reference to the "Abandon all hope ye who enter here" sign hung at the gates to hell in Dante's Inferno, this implies that Wyatt views the world outside the Funhouse as hell.
But the Funhouse is a figment of Bray's imagination, a safe place where he hides from the defeats and loneliness he feels. This means that he sees everything outside of his own head as hell, and he's happy to stay in there. The faults of his past are personified by puppets that he can take his aggression out on, and that aggression itself is personified in The Fiend that Wyatt summoned from his own darkness.
Here's what he had to say just before introducing us to The Fiend for the first time:
"There's still a lot of darkness in this old noggin. This time, I learned how to harness it, how to control it."
I believe the secret to defeating The Fiend is to banish it back inside Bray's head where its darkness can dissipate once more. And, there may just be a way to do that.