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.
The Alexander Goatstafsson achievement is located on the left side after the big field if you start at the starting point. Head to the location and you will find a red circle on the ground and some peoples around. Go inside the circle and fight against the other goats, after 4 rounds the achievement unlocks. You can leave the red circle or kicking out, is not a problem, go back and keep fighting.