Edit: Added a working link.
The first major battle on Semper Fi or Die is where you will need to get this achievement. It occurs around the statue of liberty up to the building you seek cover in mid-level.
All four soldiers you start out with need to survive. The 5th and 6th that you encounter just after the statue of liberty are of no importance.
Once the level starts and the pistol is handed to you, stay on the left side to pick up a demolition scar to increase your firepower.
- Replay the mission on easy/normal
- Do NOT use cloaking, you'll want to attract attention while having maximum armor up
- Take note of the blue dots and make sure there aren't any ceph close to it
- Scout ahead, don't stick with the other marines. Make sure you kill as much as possible in advance
- At the same time, don't go too far, you'll want to keep an eye out on your team
- If you have all four alive after a checkpoint and lose one afterwards, you can simply reload the checkpoint and still gain the achievement.