It's fun to play and it s hard to master :D well done!
I would add:
-A score: It would help me to keep track of what i'm doing
-"Damage boosting":it means that if you get hit, you get some kind of invincibility for a sec. That would help me (the player) to feel less defenseless when I get cornered with spikes .
-Upgrades: It would be cool to "buy" upgrades for the player to choose, more damage, more speed, bullet types, more "damage boosting" time, etc... It would add a level of complexity and it would be more engaging for the player to see things that can be earned to ease the game.
You have a good game here. Thanks for making it! ;)