Not a bad try, but can be a lot better.
Not bad on how you tried to imitate Metal Slug while having a slight bit of originality, but it didn't really hit the mark.
Unfortunately, there were quite a number of flaws.
1) The riflemen. They spawn from the edges of the screen where you couldn't see them, and the very moment they spawn, they can fire at you immediately. They can also fire at you in mid-air.
2) You know the truck that fires lots of missiles into the air which then fall down back at you? They have a fixed pattern and I could easily stand under the truck and shoot at it repeatedly without having to move.
3) Many of the attacks can be easily dodged by simply crouching. You could also stand right at a vehicle's position and just keep shooting it while it could not hurt you at all. I was also rather disappointed to find out that missiles cannot be destroyed by shooting.
4) Lack of variety. Maybe I missed out since I stopped playing at who seems to be the boss standing of a UFO-like platform, but I see very few types of enemies and most of them can be eliminated with the few standard methods mentioned in 3.
Could be better, but it's a good try.