Drive a vehicle up to Garcia Point and drive between the legs of the statue. If you have already eliminated the Los Muertos, the statue is not visible, so go to the Garcia Point supply point, go forward and take a left. You should be on a slanting incline and there will be a Jeep (vehicle) somewhere on your left. Take it up one more level, to the point above you, get it, and your achievement should unlock.
Alternatively, you can go to Cowell's building in Shai-Gen, bring a car into the elevator, and take it up. If you are in the vehicle, the achievement should unlock just before you reach the top.