Really cool concept! Killing UFOS was really satisfying, that was well done.
I think a tutorial, even in the game page, would add a lot. I destroyed every UFO in the first level and collected every gem, yet nothing happened afterwards and I don't know if that was the end of the game or if I am doing something wrong. The art was very fitting, and your concept is great. I think with a little tweaking of the player movement to be easier to control and some more content this would be a really great entry. I think this concept could also work really well as a game where you kill waves of UFOs as a cow, as most of the gameplay is the combat and that is definitely the most satisfying part. Great job on the game, I had fun!