Far Cry 2

You should get this through the story.
However, there is a bug (unfixed 2 years later) that might prevent you from receiving the mission that leads up to this. Known as the 27% glitch, Prosper Kouasse might be unwilling to give you the mission. In my case he just stood there doing nothing, while he's supposed to sit down in the couch and start talking as soon as he sees you.
There are two solutions to get past this problem:
-either reload an earlier savegame before Prosper glitches out (you might not have an earlier one, try to do so from now on as there might be more glitches like this later on in the game)
-If you have one available, do a buddy mission from Mike's Bar. Apparently Prosper gets reset once you complete this buddy mission, and you can continue your game. I was about to give up on the game, but this fixed it for me.
If you have no buddy missions left, try to go and rescue some more buddies, there's 3 you have to find yourself. refer to "The Third faction" achievment for their locations.
If you have no buddies and buddy missions left, I'm afraid you're out of luck...

Once you subvert six main story missions or complete three for a single faction, you'll have to defend a hut. Carefully pinpoint and drop the foes outside. After a reasonably long wave, you'll eventually be knocked out. You'll wake up in a sand storm. Here, you should follow the direction of the wind. It's a bit confusing, but if you follow the wind, you will definitely make it to the border house. A cutscene will follow. Eventually the man talking to you will be subverted by another enemy. Now, follow the on screen prompts to get up. From here, simply follow the main road south into the Southern Territory. Once you hit the loading screen, the achievement will pop.