Very cool game - the visual designs for the bosses make great use of the low-poly style. I also really like how distinctive the phases are - I love how the second boss fight escalates (haw haw) and changes the arena. Shooting green projectiles for health is a nice touch, too - it keeps you in the fight without being too easy.
I couldn't get past the last phase of the tentacle boss, though. I tried for like 30 minutes, to your credit. There's just so much going on, and that spinning attack is super difficult to avoid.