Great job! The gameplay is fun, and the particles and the post-processing effects are also really cool!
- The pixel-size is inconsistent, some pixels appear larger and some are smaller. The player sprite's pixel size differs from that of the gun and the environment. This makes the graphics inconsistent and can be fixed.
- The game doesn't really fit the theme. It would be cool if the theme was inserted in the game mechanics rather than the story.
- Give the player a reason to not shoot, otherwise the player just has to hold the left mouse button and there is no reason to let it go, not saying that it is necessary to have such a thing, but it could make the game a little more challenging as the player now has to make a small decision to shoot or not shoot. You can add recoil or limited ammo, maybe the player can't move quickly when shooting, so the player can decide if it is important to shoot or avoid enemy bullets and then shoot or not shoot accordingly or something along those lines.
Overall, this is a pretty fun game, but can be a little more challenging.