Once you gain control of the character, you goal is to complete the level without being spotted and shot at. If you are spotted, you can remain in stealth if you kill them before they shoot or inform other guards. During the first section you want to make your way through the forest by crouching and avoiding enemies which can be seen on your handheld radar. Feel free to kill any enemies via headshot or melee if you are close to them and they are alone. You will then meet up with your team where you will rescue some allies. From here on out, it is very important to listen to their instructions and follow them. Hide in the waterfall, crouch through the tall grass, prone to hide from the chopper and then finish the level without any alerts to get the achievement.
This one is in Mission: The Hunted. The first area is easy, but the part with the plane wreckage can be tricky. Go straight down the middle (close to the plane wreckage), and don't stop for anything. You should make it to your buddies and they'll say 'good job' to let you know you did it correctly. The rest of the mission is easy to remain stealthed.
Here is a video guide: http://youtu.be/6PRUQXMm6I0
-The most challenging part it the beginning when you are by your self. I recommend staying left and if you ever have a guy get to close to you just prone down in the bushes. Once you meet up with your troops just stay with them and follow what they say.
I was spotted several times and immediately paused the game and chose "Last Checkpoint" and still got the achievement. Achievement pops after you jump off the waterfall and your boat shows up at the very end of the mission.
I spent atleast 10-15 minutes crawling through the first section and got through unseen, my problem arose when I was following everyone and they say to hide behind the waterfall. I crouched directly behind Merrick(it wouldn't let me go prone) and didn't move but the enemies spotted us and of course when I go to load the last check point the check point occurred right when we were spotted...so frustrating