Meh
Not a super fun game, but its a good concept. You could easily modify it with a few more features and make it even better. I suggest adding levels and maybe ammo you have to shoot or you run out. The combo thing is useless. You don't get any extra points for maintaining a combo, and your points don't double when you shoot too at once (examle. each ghost is worth 10 but if you shoot 2 you get 40 points) Lastly, maybe make it so the enemies you shoot attack you back and you have to shoot their shots to avoid dying. so not only are you running against a timer, you have a life bar to worry about.
As for the overall concept. Its clever, but you cant really "shoot" a ghost (though fatal frame begs to differ. But your shooting with a camera that can exorcise ghosts in that game) try zombies. Or vampires. Or something obscure, but tangible. I suggest going to a specialty game story and looking at a dungeons and dragons monster manual to get ideas. I would love to see a game where you shoot Driders :3