This is fun. But... i will rate it 4 probably. Why? Hmm... You need to describe upgrades better. Best example is exploding bullets. Text said that bullets are exploding but picture has "20%" text... and some other things. You can say that fireball changes, you can add damage upgrades, you can add how to play and say there that sometimes you need head to buy some upgrades (this was hard to understand)!
Endless mode would be so cool... just add "continue" button after boss/add endless mode where boss never appears and everything else is same.
Thanks for game! It's very fun!