Hi you made a really cool game!
On Level 3 I dont understand how i can come up the platform and how i can kill the boss with the ufo. I dont really like to jump with space i think you made the game with unity so i would change the jump to "w" and "up key" .
It looks funny and Its well designed but I think it would be better if when an enemy touches you you get immuned for some milliseconds so you dont die instant.
And I found a bug that when you stop the game and then you go to menu then if you go back to the game the game is stopped but you cant replay.
I think you made a great job with all the mechanics and the game was really fun and also challenging. Thank you.
I wish you a the best in your GameDev journey!
;)