Proceed through Mission 1: Post-Human until you leave the computer room and enter a large open area. Follow the catwalks to walkways with CELL soldiers. Kill as many soldiers as possible so they call for a helicopter for back up. You will see its searchlight on the right side of the main causeway when it arrives. Use the Predator Bow with the strongest draw to get the best range and accuracy for your arrow. Select the explosive or electric arrow, and destroy the helicopter in one shot to get the "Who Needs Rockets?" achievement.