Thank you GameGabe. The puzzles are designed to be something weird and challenging, but my design skill till need improvement. Hope my next game can get better!
As for the last level, well actually there is a weapon which can decrease 50% of the demon's health value, but you need to find a hidden place in The Dark Cave to get it. And there is also a cheat mod in the last part, where the Detective won't get hurt therefore won't get killed at all. These are the secrets in this game. You can play this game again to find them if you like, or you can check the player's guide which will be uploaded today.