I really like the concept you've got going! However, I did feel a bit cheated when some of my shots didn't connect with the enemy while clearly passing through it which definitely sounds like a bug.

Other then that I would suggest making each round just a bit different to spice things up. Maybe have some different guns for each run instead of the health/speed/power upgrades you had now. At some point you could even transition into having to fight 2 last iterations.

Oww. I just now read Coatline's comment and realise it's not a bug but a feature that the enemy was invulnerable. Please communicate things like that clearly to the player! A simple shield bubble or some sfx of bullets bouncing of metal could have made it clear that my shots weren't doing anything.

I like the physics and the bouncing is a simple effect but one that helped this game feel more alive, fun experience. Please rate my game if you find the time.

This was very fun to play, and I thoroughly enjoyed the loop concept you did! Smooth controls, fast-paced action, and the explosion FX art was awesome - and a lot of what Coatline said in their comment - most notably #1, #4, and #6. 

This is an interesting idea! I like how you incorporated the theme.

Here's my feedback:

1. It wasn't immediately clear that I had to collect all the yellow balls before destroying my past enemy.

2. It could have been easier to tell the difference between bullets and the yellow balls.

3. It took me a second to figure out what the "shift" key did, but it adds a lot more depth to the movement.

4. I like that your past self keeps the upgrades that you chose.

5. It was so satisfying when I upgraded my power enough to get the tri-shot. 

6. I found that if I went fast enough in a round, my enemy would run out of moves and idle so it would be an easy w.

7. The music is fire.

I made it to Round 14 before stopping. Overall, nice entry!