First, get the "Mortar" rebel skill from the "Rebellion Rising" quest, which is an unmissable unlock in one of the first few main quests in the Itacua province (starting province). Then, go to one of the rebel side quests where you must steal a helicopter. There are plenty of them throughout the game. Two of them are in the Itacua region. They appear while investigating intel. Alert the lieutenant near the helicopter (shoot near him), and he will enter the helicopter and try to fly away. Immediately when he enters the helicopter, request a mortar strike from the rebels in the command wheel. You must be aiming at the helicopter when requesting the mortar strike. If timed correctly, it will shoot the helicopter out of the air and you will get the "Pull!" achievement. If you fail, let the enemies kill you or quit out to main menu and try again.