Follow the “Dual Fighter” achievement in level two and make sure to not get shot or you will lose one of your ships. When level three (the Challenging Stage) starts up, position your craft near the center of the screen and hold down which is rapid fire. Now you will have your Duel Fighter shooting rapid fire right into the path of the oncoming ships. All you need to do now is make sure to shoot down all ships, and the achievement will unlock.
Please see this video for a complete walkthrough of this achievement.