Keep all marines alive during the rescue in Semper Fi or Die.
You can do this in the “Semper Fi or Die” Mission, which is part of the story. It is advisable to do this mission in the “Recruit” difficulty if you want to get this achievement/trophy.
After you get down the hill, you should be able to pick up a nice rifle and refill your ammunition. Always use the scope of your rifle for better aim to kill the enemies. Don’t forget to refill your ammo when you see an ammo box.
You can also use stealth to sneak behind and go for a melee kill which is a faster kill.
If you fail to keep all the marines alive you can just restart the mission again.
This will earn you 15 Gamerscore for the XBOX 360 and a Bronze Trophy for the PS3.