Note: This achievement has to be completed in a single run, this means you cannot reload checkpoint, or quit to the menu and continue regardless of whether you die or not. If you do die, you will have to redo the mission in chapter select.

This is achieved on Mission 6 (Treehouse). The mission is about 8 minutes long and can easily be completed without even losing that much armour or health, so as long as you are constantly marking areas for enemies with your binoculars, progress slow and steady making sure to take out everyone before moving forward so you do not get shot from behind.

If you're going to die on this mission, it will most likely be from falling off the bridges at the start or off a cliff a little further into the missions, so be weary of your surroundings. Also, near the end of the mission you will have to make you way to a fort, there will be enemies trying to snipe you which can take out quite a chunk of your health even on Easy, so make sure you scan them and kill them as before moving going across the final bridge to the fort area.

Once you blow up the entrance to the fort, there will be one or two enemies which you should kill straight away, there will also be a chopper trying to kill you but you should run forward and into the right where you will be able to interact with the door, run inside as it opens and the achievement will unlock.

See video for further help: