Maximise your driving skill and ride in the agency SUV. It should have the ability to bunny hop by pressing 'B'.
Go to downtown shai-gen. Go to the tallest building, there is an elevator inside you can take to the top. One of the bosses is found at the top so you should know which building it is, impossible to miss.
Take your SUV in the elevator. Activate the lift to the top. Climb your way past several floors of ramps until you reach a balconey looking over the plaza. Build some speed, hold 'B' and let go of 'B' before the ledge. You should remain in midair for 6 seconds or more gauranteed. When you land, you will get the achievement.