I don't know what to say. Lets start with the screen shake. There is too much. The sound of the gun gets tiring quickly. Zombies stacked and three got stuck in the ceiling so even if I did beat the boss I don't know if I would have won. I did not beat the boss. I was going to try again, but the game is an assault on the senses. The game has an aesthetic. I could pick it out of a line up. I would say the aesthetic is ugly. There are too many clashing colors. It is both bright and dark. The music has too many sharps.
My advice. Cut the number of zombies in half. Replace the music with something with more bass. Less (or no) screen shake. Remove at least one color. Make a quieter shooting sound.
This does have the old arcade game feel. You got that pretty well. The core gameplay is good. There is a lot to like it is just buried under bright colors, buggy zombies,screen shake and sounds I want to mute. 6/10. Needs work but can be saved.