Nice idea, pretty fun game, but gets a little repetitive after just a few waves. I feel like the enemies could be made more difficult without just spawning more in by having the purple enemies have an avoidance radius with the player, so they try to maintain a fixed radius away from the player to make it harder for the player to hit them. Could also add sin wave or some other function variation in the relative x velocity (velocity along the vector perpendicular to the player) of the enemies to make it harder to predict their path.