Pretty good, but there were a few things that could use some work.
For example:
*Please create real immunity frames. I was only unable to be hit by whatever I was initially hit by, and could be double whammied by obstacles I couldn't see and take enough damage to lose a life.
*Dying and starting all the way over isn't that bad since the game is so short, but if you expanded on this concept at all/fleshed it out I think it would help to create overall checkpoints at the start of each 'world' of levels.
*I appreciate making the final boss do less damage since he spams homing projectiles, but I would instead recommend having more interesting attack patterns/boss mechanics to better engage the player beyond 'I run and shoot diagonally until he dies, dodging occasionally'

This isn't to say I didn't enjoy the game, it was pretty fun!
I'm mostly giving these criticisms because I see the roots for where a larger game could come from. I'm getting huge SuperTux vibes from this.

P.S. What were those halos for? I tried collecting them all and I didn't see anything happen

Good luck on future projects!