You can get most of the stunt achievements at level 2 or higher. However, if possible, try leveling up by doing all the races so that you are a level 5 (four stars and +99) driver. Once ready, go to the Agency supply point and access the Agency SUV. Exit door 1 to Los Muertos, then drive north towards the Ferris wheel. Once there, land on the beach. On the beach are a few lifeguard huts; most of them have two ramps (one on each side). Use this area to get all the stunt achievements. If you are a level 5 or higher and have the SUV, you can use a special "car jump feature". Use this to your advantage. The only stunts that need to be done elsewhere are those that require you to jump a height of 115 ft or more and to execute two front flips. You can get the two front flips by finding an elevated area that will allow you to use the SUV to gain speed. Then, press B to jump off it.
Keep reading to easily unlock the following achievements: Car Juggler, Hazardous Hang Time, Driving High, and Front Flipper. First, play in co-op mode and use a Supercar. Using it with a level 4 driving skill is recommended, although it is possible with a level 3 driving skill. Have one of the players drive the Supercar and the other find a small car that is just roaming around. Use the bridge directly outside the Agency in Los Muertos, to the right. Get on opposite sides of the bridge and meet head on in the center lane at full speed. The Supercar should send the other vehicle flying for a long time. It is possible to get all four achievements at once. However, the Car Juggler achievement may be slightly more difficult to unlock. You do not have to actually shoot the car in the air with a rocket launcher for seven seconds. The same strategy described earlier applies, except make sure that the regular car is smoking very badly as it is sent skyward with the Supercar. If you get a good enough jump up, the last bit of damage that the Supercar inflicts on the already smoking vehicle will cause it to explode up in the air and thus earn you the Car Juggler achievement. Note: You must stay in the car for the explosion to count towards the achievement. Although this results in a death, you will earn the achievement.