Interesting concept! I found some things you could improve:
- A lot of times, clicking start game will instantly kill you. You should make sure the red obstacles don't spawn into the player at the start.
- After killing the first couple of enemies, there is no challenge at all. You can just sit in one place and shower the enemies with bullets.
- After a couple of minutes, the game lags so much it completely stops.
- It would be better if the player was auto-firing. At least there should be an option for it so players don't hurt their clicking hands.