It's a fun concept, but there are a lot of small nitpicks that could have made this way better.
The elephant in the room is the graphics, which look pretty rough. The handwritten style is cool, but putting more detail or even just neatness into the hand-written (mouse-written?) text would have been really nice. The gun looks unnecessarily primitive (even the enemies have more detail) and it just looks weird how it's floating in one place and how enemies teleport onto the screen (adjusting opacity to make enemies fade in, or having them come in from off-screen like the bugs that crawl down from the top would look a little cleaner.)
The upgrades are nice, but the early game ones are pretty obvious - upgrade the gun because you'll be using it for every enemy type except the tank, which is best taken care of with the rock but can be significantly damaged by the mud and shotgun as well.
Gameplay-wise, the worst issue is that the special weapons aren't clear with when the cooldown has expired. Often I pressed a button when the graphic for the powerup showed it was usable, only for nothing to happen. Wait a few seconds, and the weapon eventually triggers upon a button press. Surely you could fix this? A secondary issue is how weird it is that an enemy just being present causes the invasion percentage to rise. It makes more sense to specifically defend something rather than killing whatever you see in any order.
Last note is that it wouldn't be hard to make this phone-compatible. Just have the weapons triggered by bigger buttons at the top of the screen; the mud and rock could be one tap to select and one tap to aim. Let the gun auto-fire and boom, there's no reason why you would need to play this on desktop.
If you're concerned about judging/placement there's more than enough time for you to fix all of these little problems by the time the deadline rolls around.