Not bad, but still has improvements to be made
I'm also a fan of the Metal Slug series like you probably are.
But there are 2 things that didn't made me play this to the end:
1 - The controls. If you're gonna use mouse for shooting, then make an aim and make the character always shoot towards the aim. Otherwise, just do like Metal Slug and put all controls on gamepad/keyboard. The controls of this game are too weird.
2 - Dying doesn't let you continue from where you died, instead, you go back to the last checkpoint. Metal Slug fans will find this very weird, like I did.
Good job anyway, I liked it.