I don't get the main gameplay loop. When you have a sword, do you just... walk up to an enemy and mash a button? I strongly recommend you play some good beatemups like Devil May Cry, Batman Arkham City or God of War so you can learn how to design interesting melee combat. Interesting melee combat requires ability for the player to not get hit. Currently, surviving the first phase with Head Cutter is entirely random chance.
I also felt that damage and range of my Head Cutter attack was random or inconsistent, and sometimes I couldn't pick up weapons off the ground, which makes trying to throw them pointless.
In addition, once you actually get a steady loop of upgrading new guns, you are basically invincible, and the only thing that can kill you is running out of ammo - due to finding the +40HP upgrade. Ironically, that's the only thing that can kill you after first upgrade gives you your first gun. The only gun that requires any skill to use is Head Shooter, as it requires waiting for multiple enemies to line up, to use the ammo as efficiently as possible.
Big issue is lack of a difficulty curve. The game just doesn't get harder as it goes on - and if it does, it's too slow to notice. I say that every minute, the game should start dropping tougher enemies. Maybe they could get more health? Maybe they can have spears that are longer range than your Head Cutter? Maybe they're very fast? Maybe give player ability to jump, and give some enemies guns with projectiles player can jump over?
Or at the very least, make enemies spawn faster over time. Something to keep player interested.
Another problem is that enemies come only from one direction. Why? The game would be more exciting if pressing A turned the player around instead of backing away, and enemies came from both directions. Suddenly, the game would be kinda exciting!
Alternatively, if you really want enemies to come only from one direction, make it so that enemies don't actually attack you - instead, at the left edge of the screen there is something you need to protect (last human baby? Virus that can destroy robot hegemony? Resistance base?) and robots don't attack you, but run right past you to attack the base. So the goal would be to make sure that no robots make it past you.
Then there's the third solution - time limit. At first I was kind of excited because I thought that if the time limit in the center of the screen runs out, the game is over. But soon I realized it's just a combo multiplier. You could say that robots lacks power, and needs to kill other robots to steal their energy or something to survive - so if the countdown runs out, the game is over.
Any of these three solutions - adding enemies in both directions, making it a defense game, or adding a mandatory timer - would make it much more exciting.
I will give credit where it's due - all of the three stars you earned due to excellent graphics and atmosphere.