Kill the enemy inside the airborne helicopter in the park

Obtained in Act 4 - Chapter 2 where a helicopter will come to attack you. You must kill the enemy that is inside the helicopter on the machine gun. In order to kill this guy you'll need the Sniper Rifles charged shot. Now all you have to do is aim and shot and once the bullet is close to the gunner press R1 Button to detonate the bullet to kill the gunner in the helicopter. The explosion from the bullet will kill the guy. This might sound simple and easy but it might take a couple tries to get the shot right on target. If you fail or missed then simply reload your checkpoint and you'll respawn at the same location for another opportunity to kill him.