While playing the Fallen Ghosts DLC, you will need to earn skill points and unlock the explosive bolt skill. Go to your loadout and make sure to equip the Phobos Crossbow as one of your weapons. You must switch between bolt types on this gun using down on the d-pad.
Next, you'll need an enemy helicopter to spawn. This can be accomplished as you play through the game or I did the main mission "Ghosts". Instead of completing the mission, I kept killing enemies to build my wanted level. Eventually, and enemy helicopter will spawn and you can easily see it on your radar. Take out your crossbow and take out the helicopter using your explosive bolt to unlock the achievement.