Pull! Achievement/Trophy Guide
achievement/trophy is earned by shooting down a enemy helicopter with a "Rebel Mortar Strike" that Is unlocked by completing the mission "Rebellion Rising" early into the game.
Once the skill is unlocked, make your way over to the south east part of the "Itacua Providence"
to find an enemy base with a rebel side mission that requires you to steal the Helicopter. When you arrive at this location, pull out your drone and search for the lieutenant who will be standing next to the Helicopter.
Fire a warning shot at him and as he gets inside the helicopter, select the "Mortar Strike" from the command wheel by pressing RB (XB1) or R1 (PS4) while aiming directly at the chopper. If done right the chopper will fly up into the air only to be shot back down, but if you make a mistake simply quit then reload the game to get this done.
If you have any questions feel free to ask!