Of Orcs And Men

This achievement unlocks after you've killed 47 people stealthily. This can only be done with Styx. When you press the left stick in, Styx will go into stealth mode and you can walk up to enemies and assassinate them by pressing A, assuming you are not noticed by them or anyone else.

In Styx's middle skill page (between melee and ranged) is a page with the stealth skill. You can mod it with a skill point to let you walk faster in stealth or reduce enemies' detection radius, I picked the latter.

When exploring, only use Styx. He runs faster than Arkail anyway. When you spot an enemy group, stealth and start trying to get behind people. Their detection radius, even in front of them (especially with the mod I mentioned earlier) is actually much much smaller than pretty much every other game with stealth, so killing them is easy. The real problem is doing it without anyone seeing it.

Some notes...

Enemies sitting next to each other, or walking in a line, will notice you when you kill someone, regardless of what position the person you kill is.

If enemies are sitting around in a circle or semi circle, they will always see you when you kill somebody.

If you kill somebody behind another enemy, they generally won't notice you unless the enemy is very close to them.

Most enemy groups have people who patrol alone, these should be your first target. People who don't patrol but are alone should be next. Even though killing people in a group will alert the others, you still kill one of them, which will make the ensuing fight easier. So, even if the enemies are in a tight circle, you should still assissinate someone.

Enemies don't seem to notice the bodies of people you've killed unless they see you kill them, so don't worry about that.

Also, as a side note, enemies will generally jump on you once they see you. Even though Styx is weak, it's not a real issue. Arkail has a skill in his defensive tree (top-left) that makes him roar. This makes all nearby enemies attack Arkail and not Styx, so the penalty for getting caught is low.

You have way more than 47 chances to get this, I got it in chapter 3 without trying too hard for it. If you are playing on easy and intend to do a playthrough on the hardest difficulty, you can actually not worry about it at all. Assassinating people on Extreme difficulty is incredibly helpful due to the increased difficulty.