You can get this in any race by racing these maps over and over again, but if your lazy like me you can use a little trick.
I clicked Time Trial, then find Monte Carlo, then you need to select the track 'Route Des Anges'.
Drive along the track and do the first hair pin, then do the second hair pin, then if you keep driving the road will sweep left. DO NOT TURN. Just let your car drive straight off the cliff. It will keep respawning and driving off the cliff.
Use an elastic band or some tape to secure the trigger and leave it for an hour or two until you have the achievement.
Edit: Looks like the video is now unavailable. Ill leave the link in case the owner makes it visible again.
Look below for an idea.
Video created by imridley on Youtube