This game is fun! The style is great, you did a good job on that. Gameplay wise, I think maybe adding an overheating mechanic to the weapons might help making the game more challenging, since when you get multiple gatling guns or rocket launchers it becomes quite easy to get through the phases and even some of the bosses that I got to were quite easy to defeat. Also populating the world with some more obstacles and jumps might make the game more challenging and exciting. This is my two cents, great job either way!