Alright, so, the sound effects are incredible :D
The graphics are pretty good, especially the bullet effects. Though the enemy and player sprites are probably too small. And if I didn't read the story in the game page description, I'd probably think you are fighting zombies in this game.
A few times I died right at the start, because the enemies started spawning from the bottom door and I didn't notice because they were hard to spot when the screen was covered by the Gamemaster guy. Would be better if the enemies only start spawning after he disappears. Especially since they appear on top of him and it looks wonky.
Overall, the game could use more of a "horror" element. It's more just silly than anything else. Making the player feel guilty for all those "enemies" they end up eliminating would probably be the way to go about it.
Anyway, it's overall a good archievement for 48 hours development time, especially since you were apparently learning how to use something new, so good job!