You will need to be in a level with your Rocket Jet Pack Unlocked and you will need to Press and Hold
shoot while flying. The best spot to get kills for this is on 2-4 Breaking Camp. At the start of the level destroy all the UFO's via the Turret. Once you do that and get a cut-scene with the soldiers dropping from a transport. When you return to the game you will be flying right at 6-7 Foot Soldiers. Press & Hold
to shoot your guns and kill as many as possible. Once they are dead go ahead and kill yourself and repeat the same process over as many times you need until you unlock this achievement.