This is a pretty fun game, not sure why it doesn't have more ratings. Good job nonetheless.
I thought the boss battle was good. Unlike the previous comment, I would say omitting the explicit instructions about how to actually damage them is the right call, because it leaves the player with some sense of discovery. I would say the better approach is to have the game feel more juicy, which means adds more effects when things happen. For example, it would be much easier for players to understand how the boss works if the crashing of smaller enemies into the boss has a much more dramatic visual effects (flashing, stopping, shaking etc.).
On that note, many other things could use more juice as well. One thing you were sadly missing is sound effects when firing bullets or when it hits enemies. The side UI could use flashing etc. when their values changes.
I did also find the game somewhat trivial with the strategy of just spamming bullets while moving around constantly. But I think this works as a 1st level or tutorial level. It would have been cool to see harder levels.
Overall I say this is well done, especially for a solo jam. Great job!