Since you mentioned it may be for Halloween, I'm guessing it's main purpose is to frighten the players a little for fun? :P
If so, be sure to add in some scary sound effects, and haunting background music (in Slenderman, what got me at times was the laboured breathing of the Player). Lighting is important too, in my opinion. Since the Player is holding a flashlight, it will eventually run out of battery according to reality science thus the area that can be seen clearly will diminish and reduce in size? A few spare batteries here and there -- scarce, but there for the Player to find?
As for killing Slender, how about distributing the bullets here and there? Instead of looking for notes to 'pass' the game, the Player has to find bullets to use it against Slenderman? They can choose to find one, see him, shoot him, anger him, dash for it, and repeat (with each shot angering him more and more, making him even more active/faster?). Or they can save the bullets they find, see him at times and lurk around to avoid him, and when they get enough to shoot him all at once, thus killing him? Just my thoughts, don't take it to heart if you don't like my opinions. :0
Saving someone, though, sounds like a drag. Could be a side-quest, for another medal. :D
Whatever, I give waaay too many opinions on someone else's game. It is a good idea though, this game, so be sure to upload it before Halloween if you can. :)
rollflasher
I think it's a good idea this game
Lenke
Fun for 3 minutes if you don't have lag, and you are not a Graphic Tester...