Like the mechanics where the demons is completely invisible, the music, art and animation was NICE, a few suggestion:
1) I think the player has the "shoot anywhere around the screen" this makes some level a breeze and also the demons all take one shot to kill and a restricted area where they can go so the train only problem was that the player has too many "safe spot", so why not make it that the player has to have line of sight in order to kill, and make the demon faster similar to the red tall ass demon after the train level, or you know giving the "shoot anywhere around the screen" to a specific gun like a sniper rifle and have to exploit this to finish a level, maybe an enemy that shoots or move through walls, or have to hit a level that's on the other side of a wall
2) sometimes you can shoot twice and only have to reload once, if spammed, but this give me an extra bullet to play with, so fix?
3) the final level where the red tall ass demon is, feels a bit too easy maybe add in some gun skeleton? or a move of the red hooded guys
4) MORE GUNS like different guns for different enemies like a gun that shoots out light in one direction in a level that is COMPLETELY dark, or a gun that pour water to souk demons in water so you can have a general idea of their position from the dripping water
final comment: F*CK YOU LET ME SHOOT THE BOSS XD