MudRunner

Start up a Casual single player game on the Island map. Switch into the A-469 (the small Jeep) and start heading south along the dirt road. Try to stick to the edges because the middle of this road is pretty deep and you can get stuck easily. Use the winch to pulll yourself out if you have to.

Keep heading south until the road curves east to meet up with the paved road. Keep going straight off the road and start heading up the hill. Keep going until you get to the top where you’ll find a lake. Turn the wheels side to side as you climb to keep the Jeep moving. You may have to climb in a criss cross pattern in the steep places.

Once at the top, turn left and head down toward the paved road. Be careful as it’s very easy to roll your car and if you don’t keep on the brakes you will do a lot of damage as you pick up speed. If you do crash badly, use the Recover in Garage option to repair the damages. From there, head back along the paved road to the block post.

Once at the block post, start heading up the hill to the east. Same drill here, turn the wheels side to side as you climb and once you reach the top you’ll find another lake and get the achievement.

Here’s a screenshot showing the path I took up the western mountain and across to the eastern mountain.