Far Cry 2

In the Northern Territory, you must complete a mission for the U.F.L.L. and A.P.R.. Right after you complete at least one mission for both sides, this achievement will be unlocked.

These two achievements (Playing both sides... & ...against the middle) are essentially story-based so they're hard to miss. In both territories, you could do missions for only one faction, the APR or the UFLL. Eventually, however, that faction will stop giving missions. This forces you to switch sides if you want diamonds, this achievement, or to finish the game.

The achievement unlocks right after you complete the mission for which you switched sides.

This one is pretty simple. By default, you start on the UFLL. In their building, they’ll give you a mission. When you head outside, you’ll receive a phone call from one of your buddies. Accept it and you’ll receive a buddy objective, marked blue. Your buddy wants to subvert your current faction’s mission. Simply head to the blue marker and follow the directions, killing all enemies when needed. Once you complete the final objective, you’ll unlock this achievement and the achievement for completing a buddy mission.

When you first start you'll only have access to a mission from the APR, complete this and when you return to Pala you'll have the option for a UFLL mission - complete this mission for the achievement. Warning - if you're going for the "subvert all missions" achievement you'll end up in a few firefights where things can go wrong very quickly - so be prepared. You'll have to destroy an ammo supply (which is a hidden truck under a bridge) only the mercs will be out to stop you so my advice to draw them into you and create a bottleneck (the northern entry point has a ravine down the left hand side) so they can't come from behind.

...oh, and watch out for the guy with the RPG on a platform at the northen end!