I enjoyed playing this game! I completely forgot the "leave one enemy alive" premise when I first got into the game, but the second I realized it become a lot more challenging and more fun. In order to get the highest score you can, you have to leave at least one enemy from a "batch" alive, but by doing so you end up having to dodge more enemies and projectiles along the way. The "don't hit face" enemy also made me smile, too: I loved the silly little dude. As mentioned in another comment, the fact that there's no music for the final boss is sort of jarring, especially when the rest of the game has that catchy tune behind it.
Overall, great work!