For this achievement, it asks you to beat any mission in co-op mode. Co-op mode is local exclusive, meaning there is no online mode. In other words, you will need either a local friend or a second controller (if doing alone).
The real difference in co-op mode is that if you're playing as pilot, you'll take control of the chopper and use machine gun. As for second player, playing as gunner, his crosshair is coloured yellow and he is makes the use of both machine gun and missile (the pilot can't use the missile in co-op). Lastly, you can't use lock-on but you'll pretty much aim and shoot most of the stuff you see on screen without the help of it.
So, you now know how the co-op mode feels for you. Play the Tutorial mission on Casual difficulty, and beat it. Once finished, achievement pops. Take note that only primary player can get the achievement.
Other solution: If you're brave enough to beat Mission 13 'Stomp the Snake' on Expert difficulty in co-op, you will earn both this and 'Parselmouth' achievement as well.