This is a story related achievement and cannot be missed
Once you start the mission you will come to a little camp. After you see that no one`s there Prophet will prompt you to go to a city and rescue a hostage. Just follow the main way that leads from the campsite. You will come across some outposts and you then need to cross a bridge to get into the guarded city. The hostage you were looking for is located in a huge building in the city. Once the cutscene ended the achievement should pop.