The idea of the game is great, but it has a lot to improve on.
I suggest you adding animations for when you hit a monster or monster hits you (like a poof/explosion or something)
Add levels
Add some kind of in game credits so that you can buy skins, buildings, monsters, upgrade player, npcs etc.
Make the infinite mode harder as you progress, so there would be a point in buying upgrades.
Sorry for the bad English