Interesting concept! Is it intended for the player to fly backwards after getting hit? Right now I'm getting teleported, and it feels strange.
What you can work on in this game is more visual feedback, like when I hit spacebar to start a wave, and I don't see anything happening immediately (because the enemies are still walking forwards) then it's a good idea to notify the player through other means, like UI, or a graphical effect. Or something like just making sure enemies immediately walk into vision.
Another thing I didn't really understand is grabbing the ammo from the box. Sometimes i press the button for it once, and i press a second time, and it doesn't work, then i press a third time, and it does! Is there an internal cooldown? If so, either change it so that button presses always get the expected result, or notify the player of the cooldown through visual feedback!
I think that's all my feedback. Good job on making a game in 7 days! It's really hard, and you did a really good job!