To get this achievement you must finish the second mission on the story mode. This achievement can gained in either solo or co-op. I would recomend playing the story mode in co-op mode as it is more fun and will help you gain a better understanding of the game.
In this mission you will be defending an oil rig from rebel forces. When you start you will have to fly over to the defnse point once there you will recive a new objective. This is where the defense will start, you will see a green ring around the base on your HUD and your TAC-map. When an enemy breaches this circle it will turn red this is usefull as you will know when even the smallest enemy has entred the ring.