This achievement is a complete pain, as not only are you limited to not being able to kill enemies, you must also finish the missions without an enemy being alerted to you. What makes it so bad is that out of all of the missions in the game, there are only five missions where it's possible to pull it off. Every other mission in the game at one point or another forces you into a firefight, which automatically alerts all enemies to your presence.
The five possible missions in the game where it is possible to complete without alerting enemies at all are as follows.
Bug Al-Samad Airfield
Intercept Nasiri the Weapons Dealer: This mission is very difficult to complete stealthily, especially the final room, due to the fact it's so early in the game. If you cannot pull it off, you can try to play the other missions and get the achievement through them.
Prevent Surkov's Escape: Must make a deal with at the end of the mission with Surkov, or you will be forced to fight.
Bug CIA Listening Post
Investigate Warehouse District
There are many things you need to keep in mind when focusing on stealth missions. Here are some tips to help make it easier on yourself.
-If someone sees you, just load your last checkpoint. You do not need to reload the entire level, so don't worry about that.
- If you plan on getting this achievement, you can make it much easier on yourself by putting 5 points into the Stealth skill right away for Master Awareness. Awareness is a very useful skill, as it will not only tell you where all the enemies are, but it will also tell you their status. Blue arrows above means they are normal, yellow means they are investigating, and red means they have been alerted to you. Many people believe that if the enemies status ever changes from normal, that they need to load the last checkpoint. This is not the case. As long as an enemies are not fully alerted(red arrow), you're fine.
- Never, ever kill anyone. No matter if you knock someone out or kill them, they will be down for good, so there really is no point in actually killing. That means, there are only two ways you can properly take care of enemies. You can either use Tranquilizer Darts, or Take down's.
- Pay very close attention to what armor you are wearing. While heavy armor increases your endurance and allows you to carry more items, it also makes you walk heavier. Whenever you do a mission solely for stealth, stick with armor that does not decrease your sound dampening. Endurance does not matter, as if you play the mission right, you don't need to be able to take more hits.
- As well as heavily investing into Stealth, Pistols should be the very next important thing. Pistols are the only firearm that are capable of not killing, via Tranquilizer Darts. As such, it's the only way of knocking out guards without getting close for a take down. On top of that, using Chain Shot with tranquilizer rounds will allow you to take down multiple enemies all at the same time. At the beginning of the game, you are automatically given a silencer, make sure you put it on right away!
- Using gadgets for a stealth mission not only costs you extra money, but is a huge mistake, save for the Sound Generator. Even still, the Sound Generator is very expensive, so it isn't something you should rely on. Every offensive gadget aside from the Shock Trap kills, so you cannot use them. Even the Shock Trap isn't good, as after it activates, it doesn't knock an enemy out right away. For a few seconds, they will be stunned, and during that time, they will automatically be alerted to you, even if they didn't see you, forcing you to have to reload.
- If anyone ever sees a body lying around, they will automatically start investigating, which can be a pain to deal with. Unfortunately, you cannot move bodies around, so this can be a huge pain if you knock an enemy out too close to another ones pathway. There is, however, one way to avoid this for the most part. As soon as someone's been knocked down, make sure that you move the camera around so that you cannot see them, and start moving away from the body. Stay away for about ten seconds or so, and when you come back, the body will have disappeared, leaving no chance of someone finding their fallen comrade.