You'll get this achievement after completing the chapter entitled 'Last Stand'
After the driving the Junker bit in the game, you and Dom re-unite with Cole and Baird, and must repel a huge Locust attack. However, this is surprisingly easy, even on Hardcore.
I say this because there are two spots in the area which respawn longshots, the game's sniper rifle. One is in the building near the Junker you just got out of, and the other is at the end of the bridge-like platform where Dom, Cole and Baird are normally seen fighting.
Stock up on sniper ammo, and take cover at one of the windows where you get the longshot. There, it becomes all to easy to take out the Locust drones. Don't worry when the first Boomer comes along either, because three sniper bullets to his head will kill him, just ensure you're under cover when he fires his missile launcher at you.
Once you've dealt with the Boomer, a couple more emergence holes will appear, just stay at the windows and take them out. Finally, a huge emergence hole will appear, and 4-5 Boomers will come along. Just ensure you're in good cover and shoot them each 3 times to complete this chapter and act, getting you the achievement.
Side note: If one of your team-mates become wounded, you can EITHER hope another of your team mates will heal him, or you can roadie run to the wounded soldier and heal him yourself, then roadie run back to your windows.