To complete this trophy you must have access to blackwater and have the mission "An Appointed Time" complete. There are a total of 20 buffalo that roam the great plains. Every time you pass the whole group you can kill between 4-5 buffalo before the herd runs off. After they run, you can sometimes catch up to them and pick them off. Keep in mind that the smaller the herd gets the tougher they are to find. To track how many you have left to kill, you can go into the stats menu and look up how many buffalo you have killed and that should give you an idea as how many are left in the great plains. Once you have killed the last one the trophy should unlock.