This was an interesting take on the theme. The shooting and aiming felt pretty natural, and I liked that you used particles for when an enemy dies. The gameplay is simple and straightforward, and generally you did a decent job.
This could have a lot of potential with some more polishing and balancing though. Your graphics, though premade, have different pixel sizes which makes the art style a bit inconsistent. The enemies also, like mentioned in previous comments, have a bit too much health and spawn a bit too fast. If you want this game to be infinite, it would be nice to have the difficult ease in with enemies getting stronger and spawning faster the longer you stay alive. Also, it seems that the quit button doesn't work (Since you used unity, the only line you need to close the game is Application.Quit(); )
This is good work for a jam submission, and I'd love to see this expanded on in the future!