The achievement is located on the first map called "Goat Ville". I created a video which shows all 19 achievements that can be obtained on this map. In the video I always start from the spawn point and go to the achievement location. The spoiler under the video contains a timeline with all achievements.
For the Boulder of Death achievement jump over the fence in front of your spawn point and turn right, cross the street and you see a red house. In the garden are some people which give a party. Go forward up to hill and you see a huge stone, headbutt the stone and the stone rolled down the hill into the Party.
After that you can obtain the next achievement which required the stone.
Lick the stone and cross the street again and head to the field. Here you see a circle raceway. Put the stone on the lane so that the car hits the stone.