This achievement takes place on the mission Going Dark.
If you get spotted, if you're fast enough you can
revert to last checkpoint. When going for this achievement, I highly recommend that your
turn the subtitles on so that you can see when the enemies are speaking. This is particularly useful when trying to find if there’s another person patrolling in the area. Another thing I noticed is that when you go to a different area, some of the enemies will be trying to
contact the people from the pool area for example to see what they’re doing. If they get no response then you've killed enough people in that area.
This achievement takes place after you repel down the mountain from Captain Price. First off, I recommend heading to the Pool area first as it’s the closest. Head right from the area you drop down and you’ll see the pool. Jump onto the sandbags to navigate over the wall and you will see two enemies by a
lit fire, take them both out. Next you will want to deactivate the power for the pool which is found
nearby the fire. Once you’ve done that jump on the dumpster to get onto the
roof. When up here you will have a great view of the enemies from inside. Take out the enemies from the roof and jump on down. Once down patrol the building for any remaining enemies. Then interact with the hostage.
Next make your way to the
Courtyard area by heading out the entrance near the fireplace and turn right after you go past the
small building. Now by this point a few enemies may be walking to the
pool section. I recommend taking them out just in case as they could potentially find one of the bodies from earlier. Continue up the road and you’ll come across the Courtyard.
Next you will want to
deactivate the power to the Courtyard which is directly in front of you. Also shoot out the
streetlights if possible as it’ll make navigating around easier. Head on inside through the window on the left side of the building and make sure to not get spotted and take care of the enemies inside. Also, by this point an
enemy vehicle will be patrolling the map so make sure you are out of sight when it approaches. Before leaving the courtyard, double check the building and the outskirts for any other enemies.
Head from the Courtyard to the Church by
following the path via the left side. The Church gave me the biggest trouble as there
isn’t many places to hide whilst outside in the graveyard. When in the graveyard I recommend that you
take your time to make sure that an enemy isn’t close to another enemy, reason being is that if you kill them when they’re near someone else then they can get
suspicious of your presence. The
power switch for the Church can be found in the graveyard on the left side.
Take your time navigating the graveyard, then head on inside and you will see 2-3 enemies in there. Take them down whilst inside. By this point I thought I had killed all of them so I waited around a bit checking the graveyard and some enemies from the road walked into the perimeter. I dealt with them and the achievement unlocked.
If you are still struggling, feel free to use this as a reference.
Call of Duty: Modern Warfare We Own The Night Achievement Guide