This achievement is part of the storyline and can't be missed, provided you complete the mission.
There are plenty of YouTube guides on how to complete this, but for those who don't use them, a few hints and notes for the mission:
1. There are a lot of enemies in a very close proximity in this mission, both outside and inside the fort.
2. Once inside the fort, kill all the enemies you can up front as it will make coasting to the end of the level much easier.
3. After you climb the ladder (helping the prisoner escape), you'll be on top of a building with your mission to protect the prisoner. Don't use this time to tag enemies, just shoot those who are closest to, and shooting him. If you let him sustain enough damage, you'll need to replay - so time is of the essence. Like this game has been, you will see the yellow/red circles over your enemies - nobody at the end will see or shoot at you.
Once you clear the path and the prisoner escapes, the achievement will pop at the end of the level. If it doesn't pop for you (didn't for me), check to make sure you're connected to Xbox Live. Once you are, the achievement will pop.