For working it you need a outpost with a gyrocopter and, buy the wingsuit. I've use gyrocopter of the first outpost, you have water at 500 meters on north. Up as high your gycopter can, press
then
3 and
. If you finish in water you'll never die and can fly more. The trophie will pop when you fly.
Edit: like CrucialVelocity said in comments, its a mission later in the game you need to fly 1000-2000m with wingssuit. This mission start at airport on north's map. It will be effective to use my trick after completing that mission.