To get this achievement, you have to at least be on Stage 2. The green ships at the top of the screen that have to be shot twice to kill, will occasionaly swoop down and shoot a "tractor beam" that will take one of your lifes it hits you, but the life is carried upwards with the ship. Shoot the ship and the life before you kill the rest of the enemies to destroy the extra life and unlock a surprisingly traitorous achievement. Be warned though, if you don't shoot the life before shooting all enemies, you don't get it.