The 'Maximum Air' trophy/achievement is unlocked if you be airborne for 4 seconds and more in a vehicle, also you should have a safe landing(without dying). People find it difficult to unlock as this achievement asks to stay Maximum in the air. With this guide, you will unlock the achievement with no pain.
How to Unlock 'Maximum Air' Achievement/Trophy
As we mentioned above you need to stay in the air for more than 4 seconds so you should find a High peak to jump from and also take care of the safe landing. Check this location 'Camp Polygon' in the Map. This is one of the highest peaks in the game. Go to the Location.
Now Climb up the Peak and make your jump from one of the peaks and land safe. Check the Video below for more appropriate jump view.
Note: Do not try to fly into the camp.