I really like this one. It's tough, but the power-ups help. Buying power-ups seems a little unintuitive. (I'd have thought it'd be A to buy the power-up and B to exit, rather than Enter and A)
The music sort of sounds like a 2000s Flash game more than a Gameboy game (though I know that sounding exactly like a Gameboy game was not required, but it just had the wrong vibe for the setting), and there were no sound effects.
Also, last issue was odd hitbox on the player's attack; the animation lasted longer than the actual effect. The enemies could walk through it after the first couple of frames, even though the visual was still on screen.
Was the last room (at least, I think it was the last room) meant to be meta-commentary? If so, very clever.
Overall, good game. I can't wait to see how it develops.