This achievement requires a fast sports car, like the De'Leo Stiletto, which can be found (rare) randomly driving around in the Frisco Fields region (north-west of map). Other high-end sports cars also work. There is a construction site in the north-western corner of Frisco Fields (by the water). It has a large ramp made of dirt. Start driving from the end of the map to the ramp at the construction site. With enough speed, you will be able to reach 50+ meters easily. The car must land on its wheels to get the "Testing The Shock" achievement. There are many other ramps all over the map. Later in the game, you can also unlock upgrades that improve the speed of your vehicle. Thus, if you have any trouble, you can wait until later in the game.