How does Sombra’s “Power Outage” achievement works

You are only allowed to kill one human enemy in the entire game and that is the Flamethrower guy at the end of the game in D6, everyone else you have to either knock out or avoid. Be careful during the Contagion chapter as enemies have a habit of running into the fire and dying on their own, if this happens then be sure to restart the checkpoint. Always try to be stealthy, knock out the guards and going through Contagion quickly but carefully will help to ensure that you get this trophy. Be sure to check out the stealth videos for Separation, Facility and Revolution if you're having trouble making it through those levels.

• Chase: The beginning area can be somewhat annoying because crouching down will still allow you to take damage and the medkits don't seem to help much when you're being shot. I recommend having 4-5 medkits before starting Chase if possible, I also found that crouching and standing up every few seconds kept me from being shot as much then if I just crouched the whole time. Once on the train try to hide behind boxes as needed but know that they can shoot through most of them so try to keep moving, there are however some metal boxes you can hide behind that they can't shoot through once you get to the enclosed train car.

• Red Square: When you enter the square you will be surrounded by Reds on all sides of you. Your best bet is to hide behind the tall barriers until the guys start to walk up on you, disarm all of them and continue to hide until Pavel gets bored and opens the gate. As soon as he does run at the gate and disarm the guys. Note: You have to shoot at Pavel to continue the Chapter and make it up the stairs before you can decide his fate.

• D6: Stay crouched behind one of the small barriers and let the AI kill everything, there is a sniper on the wall to the left that you can use to shoot at the tank with. Note: There are ammo crates all around and there is a Minigun on the last platform that you can use to shoot the Flamethrower guy with.

I highly recommend following this link if you're having trouble: http://www.playstationworld.co/threads/3186-Metro-Last-Light-Shadow-Ranger-Guide

Note: You can kill Mutants just no humans so Spiderbugs, Demons and Watchmen are okay to kill. Also be aware that killing certain Mutants will affect the ending you receive.

Source: http://www.ps3trophies.org/game/metro-last-light/guide/

After repeated attempts, this proved to be something not likely doable when facing AI opponents alone. It turns out that some achievements can be earned in local multiplayer and, thankfully, this is one of them.

A simple strategy to win this achievement is as follows:

• Set Monsters to the lowest amount to avoid interruptions.
• Optionally set Minerals to the highest volume to make mythril easier to find.
• Optionally set Relics and enemy AI to the lowest values to make it less likely that an AI opponent will score from Relics or will obstruct you.
• Play a map with easy access to the Deep level, such as Cliffs of Blandness, picking the lower starting point. A two-player map will be the quickest because there are fewer players that need to take turns.
• You can try lowering the number of turns to make it a bit easier, but the default should be fine if you are in control.
• Set two or more of the teams yourself to begin the match in hotseat local multiplayer where you are controlling that many teams. It's not as much about quantity as movement in this case so having one or more Scouts will make this easier to avoid mining the wrong mineral.
• However you go about it, be it controlling all teams or facing some AI, prevent the other teams from scoring much if at all and have at least one unit on one of your teams collect only mythril, making sure not to pick up or turn anything else in. Be careful of mining the wrong minerals, especially where the game obstructs the display of the mineral type.
• Once you've turned in enough mythril to score more than the other teams, you can pretty much coast your way to this achievement.

I did not have the opportunity to test whether turning in relics invalidates this achievement because I simply made sure not to pick any up. I am confident that monster looted minerals that are not mythril will count against this achievement.