Should be free if you own the DLC pack. If not, buy the jeep. From the Horizon Festival, the track above it goes out over the water. Head there and find where the track will let you drive off of the track without the barrier. Drive up where you have some height and not right on the beach. Line up the jeep where both tires go off the side at the same time so you'll start rolling over the edge. The achievement will pop even if you land in the water and it re-spawns you.