Liked the visuals for the enemies and the player but their needed to be a different colored bullet for the enemy to better differentiate between the two bullets.
The first 5 waves require you to move to hunt down the enemies as you don't have the spread to hit them from a far. But after Wave 5 you can just hide in a corner and spam the shot button because by now there are so many enemies and you shot tons of bullets you don't have to aim. This could be fixed by adding a enemy that moves towards the player and deals high damage upon impact making so that the player has to move around or changing the upgrades so that the player does not shot more bullets but rather more deadly bullets. This would require the player to move around and get closer to the enemies in order to hit the fast moving enemies.
Boss was good as it required you to be out of the corner but the fact that the Boss does not move makes really easy to get down it's pattern. This could be changed by copying the movement system used on the enemies and just making it slower. You could also have the the boss spawn in enemies as it increases the number of bullets it shoots. Additionally I would change it so the player does not get life points from lowering the boss's hp to a certain but rather spawned in enemies.
I know this is a lot and you probably could not implement this stuff do to the time restrictions.
Needless to say I really liked the smooth gameplay but there could still be improvements made.