FAITH

After receiving the shotgun head into the room to the left and shoot the mirror. Go back and retrieve the gun again. Repeat until you break the mirror and then proceed to defeat the boss.

There are a total of five endings all determined by what you do with the shotgun.

1/5: Shoot Amy. She can be found in the woods, shoot her and proceed to the car.
2/5: Return to the shack where you found the mansion key and shoot the stranger. Proceed to the car.
3/5: Shoot the sacrificial fox in the woods. Proceed to the car.
4/5: Shoot the deer in the woods. proceed to the car.
5/5 (good ending): Proceed to the car whereupon you will be ambushed by the demon, shoot it.

Note: This DOES NOT require 5 playthroughs. Upon completing a single ending just cimply go back to chapter 1 select and hit "continue", you'll start where you found the gun.