BAJA Edge of Control

I advise doing this one after your complete your Baja Career. Once you complete the career, go to Race > Open Class > Mogollon Falls Challenge. This should be the only Open Class race you will need to finish 1st in to get the achievement.

After completeting all the open challenge in the career , go to race-open classe and choose mogollon falls challenge and use your personall baja and upgrade it whit the best clucht , it should be peice of cake and the achievements is yours.

PLEASE NOTE: The other solutions do not point out that your starting position depends on the vehicle you choose. If you choose the "Baja Bug" you will START in 1st place, and only have to maintain 1st position for the duration of the race. This is by far the easiest way to get the achievement.
You can use another class of vehicle, but your starting position will be effected as such...
1. Baja Bug
2. Unlimited VW
3. 4x4 class
4. Open Wheel
5. Mini Truck
6. Fullsize Truck
7. Class 1 Unlimited
8. Trophy Truck

As the other solutions point out, if you have completed the career then the only "Open Class" event remaining is the 'Mogollon Falls Challenge'.
From the main menu go to Race > Open Class > Mogollon Falls Challenge
Setting the AI difficulty to 70% (easy) will make this a fairly stress free race.

Hope this helps !!