It's a complicated situation. Different enemy types move and fire in different ways, and can be distinguished by the different noises they make. Compounding this, each of your guns needs time to reload before firing again. On the plus side, the player does have a shield at their disposal, though while it is raised one can't attack the invaders.
Each level is timed, counting down, and the player's goal is to hit as many enemies as they encounter (rather, to avoid being hit by any enemies -- in this fixed position, the best defense is a good offense). At a level's end statistics will be calculated, measuring accuracy.