It's a solid game, and I like the concept, there are 2 things in particular that would make this stand out:
- Game feel: for a game about having only one bullet, that one bullet does not feel impactful. Screenshake, damage flashes and hitstops could be utilized to juice up the gameplay.
- Clear communication of game mechanics. The first time one of the portals spawned two enemies, I felt cheated. I lost life, even though I stayed clear of the portals. If there are two enemies spawning, show two portals, so the player can prepare. It's frustrating to die just because the game did not tell you that the portal did not equal the enemy position this time.