just to be clear, this is me trying to help, not me saying what's wrong with the game, I love it!
maybe you could add a cap to how much he can shoot though, because the more you kill your fingers the faster he shoots lol
you can pick up health even if you're at full, if you get hit after that, you can't get it back
the jump feels a bit stiff, like the gravity's very high (I don't know if that's the same with mega man) but I tend to jump too low a lot, maybe if you made the timer for how long you have to press your jump to go very high a little shorter? Or just decrease the gravity!
A super? For every (10?) enemies you kill you get something that does 5 times your blast damage?
The first boss (the snail one) can be easily beaten by standing on the platform an spamming X and S at the same time (in like 10 seconds)
overall, it's a VERY good game, very nice artstyle! Thx for taking your time to read this, I really thought everything through!