If the game already has a mute button, it needs to be easier to find, and if it doesn't, it needs one.
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.