For this achievement, you'll need to have the Fallen Ghosts DLC (obviously)
This one can be a little luck based but isn't too bad. To get it, you'll need to have completed the first mission of the Fallen Ghosts DLC.
After you've done so, in the skills menu, an extra tab will be open called "Fallen Ghosts". You'll need to have enough skill points (only 1 is required) and materials to purchase the "Explosive Bolts" 1st tier. The more times you upgrade this skill, the more ammo you get, although only the first tier is required to get this achievement unlocked.
Play as normal until you a) see a helicopter in the open world (shoot at it to get it to fight you) or b) attract enough attention in a mission that a helicopter comes to do you in (which I found happened significantly less in this DLC...). Then equip your Crossbow in the Loadout menu. Look down your sights with LT and press down on the D-pad to equip explosive bolt ammo, and aim it at the helicopter whenever it is in range and at a more stationary position.
With 1 direct hit, the chopper will come down and with it a nice new achievement should unlock.