This achievement generally comes with playing through the game on veteran, although if you play through on hardened you will still get this, you'll just miss out on the numerous 30 point achievements for completing the missions on veteran. It's recommended that you play through on veteran, however if you get stuck on a mission or two (I got stuck on Heart of the Reich), you can do those on hardened and still get a good gamerscore.
A few things to remember when playing on Veteran/Hardened:
1: Throw grenades back when they are close to you! Grenades have an abnormally long fuse time, so take the time to locate it and toss it back. Only throw it back when the grenade indicator is not transparent (i.e. it's near your feet).
2: Cook your grenades! If you don't cook your grenades at all, the enemy will just simply toss it back at you, and the odds are that it'll be perfectly cooked and will blow up in your face.
3: Take cover! This is a given, but if you do not take cover, you will die. Stay on cover at all times, only moving out to either avoid a grenade or to proceed with the mission. When shooting from cover, take the few shots you need to take and duck right back down.
4: Take your time! Veteran will take 2-5 times what it'd take you to complete the same mission on recruit. Proceed slowly until you get your next checkpoint.
5: Don't rely on your squad! The squad AI is essentially worthless, and you can't rely on them to do much of anything, as much as you'd like to. They are good for bullet sponges though.
6: Don't get frustrated! If you get angry, you'll get impatient and you'll wind up taking much longer to complete the mission IF you do complete it at all. Take your time, and if you get angry, go play Uno or something to calm down.
7: Find a video of what mission you're on! It'll help tons.