I really liked the idea of the reflection attack. Also the way you get introduced to it is quite nice, finding the two powerup items.
Unfortunately I couldn't beat the boss after 5 attempts. Timing the shield deploy is quite the challenge. I think it would be more fun if you would remove or prolong the timing constraint, at least for the boss since I found timing the shield deployment and aiming at the bullets a bit overwhelming. I managed to get it's health in half. Maybe some healing items during the fight would also help.