Make sure you have a rocket launcher. Take the body or car up the elevator in the Central Point building in Shai-Gen. Once out of the elevator, go up the ramp to where it goes outside. Take out the rocket launcher and throw the body or car up in the air and slightly away from the building. Then, just fire rockets at it to keep it in the air for a few seconds. Once it hits the ground, the achievement will be unlocked.
Use the following trick for an easy Car Juggler achievement. First, make sure that you have a full load of cluster grenades ready. Then, get the Agency Supercar from the garage. Next, go out door 1 into the Los Muerte district. Drive the car into the water, but not too far away. Get far enough where it is floating and you can throw a grenade under it. Once the car is floating in the water, throw as many cluster grenades under it as possible. When they explode, the car will soar high to a point where you can just barely see it in the sky. If done correctly it should stay up there for about fifteen to twenty seconds. This will not only get you the achievement easily, but will also give some enjoyment as it is the easiest, best, and most fun way to launch a car into the air. Note: This trick does not work with bodies.
