Also I'm not sure if this is intentional, but the second boss is... undefeatable? Maxing every upgrade does not allow for enough damage output to kill it. Maybe there's a trick here, but without knowing what its health bar is, I can't even tell if I'm on the right track.
I got it on my second try. Maybe if you don't buy full auto there's strategy to it, otherwise the strategy is buy everything and hope for the best.
It says you beat the demo after the second boss. I'm guessing in the full version there's extra upgrades you can buy after the first boss, so it should be more consistent.