A lot of real work has gone into this. I think development has been abandoned, but I still wanted to post some feedback.
Unless the goal is to make a game harder than *Metal Slug*, I find the enemy projectiles far too fast and visually small. Not only are they nearly invisible, but they also move at high speed.
The roll mechanic, which grants invulnerability, is a nice touch but isn't really usable for the reasons mentioned above.
The game speed could be bumped up slightly—say, to 1.25x (though you'd need to keep an eye on the speed and visibility of enemy projectiles).
I made it all the way to the boss and finished the game.
Personally, I’d prefer the auto-fire to be a bit faster, though I can see the logic behind an automated assist being slower than manually spamming the fire button.
In short, this could become an alternative to *Metal Slug*, but it still needs quite a bit of work. The genre isn't dead; players are waiting for new titles, but they rarely come along.