You’ll have to fall for a continual three seconds. Your best bet is to get into a helicopter (passenger or pilot doesn’t matter) and then wait for the helicopter to reach near its maximum altitude, you should know when it’s at its highest because you’ll begin hitting an invisible barrier. Hit and watch yourself free fall for a few seconds before impact and the achievement is unlocked.
The only levels with a helicopter are “Oasis” and “End of the Line” and you’ll either have to be an attacker at the first base or a defender at the second base on “Oasis.”
See the achievement “Vehikill” for specific instructions on piloting the helicopter. Remember, push forward on the to raise the altitude of the helicopter.