I've never seen a jam game that entirely focuses on a boss fight. It's an interesting concept, and it seems like there was a bit of a unique idea for a boss fight in and of itself. At least somewhat.
Though unfortunately during the boss there's not really much to do. It's got a really simple attack pattern that leads to very quickly optimizing the fun out of the fight honestly. If you really aim tow in you can just hide in the bottom corner and shoot whenever the boss passes by and you have absolutely no danger. Maybe if there were some attacks from, the side ore enemies that flew in, webs etc. that could mix it up a bit, but for the moment it's just a matter of focusing on the top.
Which to be fair, can be done quite well, there's just no t that much variety currently to focus on there. Maybe if there were a few more weak points of the boss or some sort of alternative action other than to move in front of it's face to attack etc. that could help?
Cause at the moment the core loop mainly consists of, stand in front of boss, move, repeat. Possibly kill some other minions, move in front. There's not much variety to that, besides the small power ups but those don't change too much of the gameplay, they just sort of work as cool powerups. Which don't get me wrong, can be interesting, they just don't mix up the game that much by themselves, since there's not really any enemies that force you to take advantage of it. The minions die to slowly for the weapons to feel that powerful on them.
Then there's not that much change without hitting the boss consistently for that to come through. Also considering the boss pretty much blocks the only attack route quite a lot, you spend quite a lot of time with your powerups just waiting.
Ignoring all that though, I do mean it when I say it has an interesting setup and a bit of a nice base for a SHMUP and a cool base for a boss fight that I would be interested to see expanded on.