Unique take on the limitation! The gameplay was really intense too! I do recommend focusing on clarity and art for your next few projects.
It was a bit unclear on what to do and how to do it. I feel like the gameplay should have only relied on one button, as it would probably be more simple to play. Also, while this might not have happened due to time constraints, I feel like the different gunners should have different corosponding sprites, as polish.
The art, to put it lightly, isn't the best. The colors also look muddy and dirty in a way (might have been intended), and it gave me mspaint vibes (especially the bus), and I don't think you should strive for that in most situations. I recommend going to Lospec and finding a color pallete that you like, and using it as a base for your pixel art. You would be surprised to see how it could improve your art.
Other than that, great job!