So this game isn't very good, but here are some suggestions for improvement so that your next game can be better:

- If you are doing a shooter, allow WASD controls in addition to arrow keys. This lets right handed people use the mouse or touchpad to aim while using the left hand to navigate.

- Graphics need improvement. A few specific points: First, the tiled textures look very unnatural. Unless tiles are part of the game mechanic, I really don't think there is a reason to use tiled graphics in a modern game. You can create one huge background image for the entire level and you won't have to deal with the seams or repetitions between tiles. Second, there are weird areas or pure white or grey. And there is a whole extra map randomly placed at the bottom. These should be easy to remove. Third, there is no reason to make things so small! I really had to squint to see that the player is a guy holding a gun. Still not sure what the red things are.

- Look up some algorithms for "steering." Steering is the process of simulating smooth motion instead of sharp, darting movements. Also look up A* pathfinding, which allows NPCs to navigate around obstacles instead of bumping into them. Right now, it kind of looks like bugs are darting around the screen. Movement speed for the NPCs is also way to fast right now. Feels very unnatural.

- Add music. If you do a web search for "royalty free music" you will find lots and lots of music that you can legally bundle with games. For example, Kevin MacLeod is a composer who made literally hundreds of songs that you can download and reuse for free. Adding music to a game is easy and really adds a lot.

- Flesh things out. Adding a small intro and/or tutorial, adding a pause menu with a list of controls, adjusting speeds, and so on really adds a lot to the game.

So good work on the first game. I don't think it is good enough for Newgrounds just yet, but if you fix some of these oddities/inconsistencies and work on polishing the final product, I think your next one could make it. I hope this advice is helpful.

Bugs:
-Gun stops working at a random points (not related to how many times you've fired, so not ammo running out)
-Whenever you stop moving with the arrows keys, the player slides with momentum toward wherever the cursor is no matter which way you were moving

General suggestions:
-Enemies shouldn't move so fast
-AI that's more than bouncing around
-There doesn't seem to be any cooldown time after an enemy hurts you, add some invuln frames
-Add effects when things get hit
-Score
-Ammo pick ups
-Weapon pick ups
-Music / SFX
-Title and Game Over screens that aren't just pure red
-Display player and boss health bars in static corners of the screen

Keep experimenting, and you'll figure it out (the health bar and probably other stuff in the process). You're on a good way to start making interesting stuff. This one itself can be somewhat challenging due to the 1 hit ;) and some ppl like that.