You must kill all invaders by using various weapons, like bombs, laser, missiles and machinegun. Press number keys to change your weapon. There are several types of enemy fighters. They shoot bullets, missiles and laser at you. So you must move up and down to avoid being shot. You can collect some shield item on your way to recover your health or become invincible.
Pay attention to a kind of cuttlefish missile for it is invincible under the protection of a kind of protective layer. Try to survive in the war.