Semi-on-the-fly remarks:

1. Controls are QWERTY-only instead of layout-independent (I have an AZERTY keyboard).
2. Ah, instantaneous shots! Not that common!
3. The fading may be a bit too long. (Nothing serious!)
4. Is there any way to dodge the missiles?
5. I like the humour (second boss line, for example).
6. The second time, ‘GAME OVER’ was still written; is this expected? Also, I did then not lose when reaching zero hearts, and taking one more hit, they went back to three. This actually persists for all subsequents rounds, including having a still missile.
7. The submachine gun makes the boss health counter display many decimals! I think less (say, two) would be fine.
8. I think the submachine gun turns out to be way slower than fast clicking with the base weapon/pistol!
9. You can just let the mouse on the boss… I think either the boss should move, or you should have to click on the missiles, so that you have to move the mouse.
10. I stopped after 7 rounds, since the line seems to stay the same.

I like the humour (including the title screen) and visual+audio style (especially the cute hero with his giga-wide eyes), but I think the fights lack some variety; I may have missed how to deal properly with missiles, though, so tell me if I did! The way the game is played is straightforward and clear. As for the constraint, you fight the same fight with possibly a different weapon for you sometimes, because winning gives you some ‘win money’, but I think the boss itself would have to vary to make the challenge more interesting — otherwise, it is literally a pure loop!