Review Time!
This review is unfortunately not going to be very nice. But I will do my best to highlight where you've done wrong respectfully, and suggest some possible improvements.
First of all, this game is conceptually not that interesting. There's two ways to approach a concept like killing a thousand enemies: you can make so you ravage through dozens of enemies at once for around three minutes, or you can make a slow, repetitive killing spree that could last for dozens of minutes. You decided to take the latter option, which leads to a very uninteresting game on its own. It would be much more fun conceptually if you killed 20 to 30 enemies per second with an extremely powerful gun, rather than having to kill one at a time in a repetitive fashion.
Furthermore, this is probably not the most suitable premise for a game jam. Making a game like this work requires some well thought-out maps and diverse enemies with diverse patterns. This isn't something that can be done in only two weeks, and it shows.
And unfortunately, not only the premise isn't very promising, but the execution also leaves a lot to be desired. The map has extremely poorly communicated boundaries, with invisible walls being all across the arena in extremely unintuitive locations. There's nothing for you to jump over, anything you could use strategically to avoid or lure enemies, just a regular flat arena with some obstacles placed across it.
Not only that, but shooting also feels extremely unsatisfying. More often than not, your shots are going to miss when they absolutely should have hit, and it's often frustrating how such a tiny bullet isn't sometimes capable of hitting such massive skulls. Sometimes you will even miss shots against the gigantic blue skulls, sometimes even when they're close to you!
And finally, the last problem I've spotted was the camera. Since it's a first person shooter, the player is in charge of moving the camera around, and that's not inherently a bad thing. The problem is that the game isn't designed to accommodate that. Certain enemies are faster or larger than others, but all of them chase you in the exact same way. This will result in you constantly walking backwards while trying to shoot the enemies on your way, which then results in you crashing onto objects obstructing your path or even enemies that were chasing from you behind. Looking at all directions at all times isn't exactly a solution, either, because then killing enemies will be far less efficient, and you're already pretty weak as you are.
I'll have to admit that I did not manage to finish the game. Maybe some of my points don't hold as much weight when taking the whole experience into consideration, but the game really didn't do a good job at hooking me.
With all of that said... how could this game have been better?
First of all, I think this concept would have worked infinitely better if it was a 2D, top-down, third person shooter. Think of something like Enter the Gungeon, or even better, Vampire Survivors. This way, you would have a perfect vision of all enemies around you, it would be far easier to move your aim around, and making proper collisions for the enemies and bullets would be far easier. Not only that, you would have time to make the game far more complex, since it would be a 2D game.
Maybe you could have even added a knife, or some sort of melee weapon to the player that deals more damage than the guns despite being more risky to use, to give more variety in how the player could approach certain enemies.
If you *insist* in keeping it as a third-person shooter, though, at least make your weapons stronger, and don't make every single skull follow you at once. Instead, make them only follow you if they spot you. Make some skulls with dashes, charge attacks, maybe there could be some skulls that stay still but can shoot you, and so goes on. Oh yeah, and please put some mountains on the invisible walls.
Game Jams are a learning experience for everyone, and I hope you can take this project not as a failure, but as a worthwhile learning process. I wish you luck in your future works.