...against the middleG 20

In part this is a story based achievement that is unlocked during the course of the game. It works the same way as the Playing both sides... achievement. There are two factions in the game, the U.F.L.L. and the A.P.R. that will give you missions. You can choose one of the factions and finish off all the available missions in the Southern territory. Once the missions are complete you can start the opposing faction missions and the achievement will pop once you complete the first missions. You can also just complete one mission from each faction to pop this achievement quicker.