It is lacking, well, a lot to be a good boss fighting bullet hell... Here's some QOL changes you should implement:
- Indicator for where the boss is at
The boss can frequently leave the screen, and it makes it difficult to gauge where it is at without just plain shooting into the dark
- Parry/dodge
A classic of any boss battler, the boss is meant to be an asshole, at least give the player a chance if they get cornered
- Direction indicators
The poison having a way to gauge where it will be at is useful, but not seeing what the trail will be in advance harms this communication
- Invincibility frames
I died SO MANY times just because of chained attacks, I shouldn't die just because I got unlucky with bullets and poison getting closely chained together