This is quite good! I particularly like how the boss looks. 

I wanted to mention, there's a bit of an audio issue when the boss is firing bullets. Presumably due to the shooting noise playing many times at once, the audio has a "peak" effect where the noise created is very high and sharp. One way to improve this is to have something separate that manages shooting sounds, and have bullets call that instead. From there, it can do nothing when called if its already playing the sound. (This method would hopefully require minimal changes)

Apart from that, I really enjoyed it; catching the upgrades was very satisfying because of the clear changes in your bullet patterns and colour. :D 

This is a pretty charming little game! It's trying to do something very specific and it's executing it mostly really well. There's a lot of really good details such as the way the powerups spawn in and the screenshake and smooth movement of the boss; whose pattern is a good mix of simple and challenging. It's pretty comfy-- I'd say this game's biggest flaws are just gamefeel: It feels like the boss has a LOT of HP, and also when you shoot him there's no flash, no hit sound effect, no healthbar, and it doesn't feel "satisfying" at all. For a while I wondered if I was even doing damage or if my shots were just dinking off his head. It's a minor thing, but it's pretty crucial since the entire game is just the boss fight. It's cute, though!