The enemies all shoot in a similar pattern and have identical behavior. This will always create safe locations in each pattern. Wait for a safe opportunity before firing back. Enemies will try to push you down. Do not keep your ship at the bottom of the screen. Remain in the bottom third area to give yourself space to dodge their shots and the enemies themselves. You will also have room to maneuver for better shots. The closer you are to an enemy, the faster your missiles will take them out.