ay, this is actually pretty fun. I do have some suggestions that can make your game a bit more interesting. (there is no intention of hate or rude criticism in this comment)
1. Maybe make it so you can get coins every so many enemies you kill, so you can perhaps buy upgrades.
2. Perhaps temporary power-ups that enemies drop which you pick up and give you a high knife velocity, speed boost, or immunity for 3-5 seconds.
Overall, this game was really good! I find myself playing it every so often. Good job! :)