Boss balance issues

I have been playing this game for the last few days, its a nice distracton and at 1.99 cant ask for much more.

The grudge, that made me rate it so low, is the boss balancing. I had no chance to defeat the boss in the 3rd stage, second boss (boss you can see in the 5th screenshot) in casual mode. Up to that point the game had been relatively easy and relaxing (what I would expect from "casual" mode) but after trying going throwgh the entire 3rd stage twice with different sets of weapons upgraded (attempted with the wave and misiles both fully upgraded) I ended giving up.

Other than that, the only other critique is the background music, that gets very monotonous and remans the same even in boss battles.

Mariachi Samurai about Radiant