GET EVEN

Mission The Offer
I found it easiest by taking guards out at range.
Although, you can do this without having to deal with many in the first place.
When you see the shipping container at the very beginning, take the path right of the fallen tree.

Follow this along until you hit a river. One guard to take out from distance with a head shot from your silenced pistol. Stick to the right wall again. In the next area, keep sticking to the right wall, there is a guard having a smoke up the hill, take him out (again from distance) and follow the edge of the map.
You'll come to where the secret door is, kill the guard nearby before he spots you. And then find the gate to the next entrance.

Follow the map left. Eventually, you'll come to three guards talking, let the guard walk passed you. Go up the hill, take the guard sitting down first. Then stealth take down the guard on the phone he was watching over. Follow the rock right. There may be a few guards to take down, sometimes they move about, other times they don't. As soon as you reach the door to the meeting, Red will say something like "you surprise me Mr Black" and the the achievement unlocks shortly after.

Taking this approach should unlock another achievement for not raising the alarm.

This achievement is unlocked by completing the 'The Offer' memory, which is the seventh memory in the game, without being spotted (yellow is fine - red is not).

My advice for not raising alarms is to sneak up on an enemy by crouching and approaching them from behind and taking them out with . The more guards that are killed, the less danger there will be of being found. If you are detected, reload from checkpoint and try again.

For this one, do not go out of your way to kill guards unless they are in your path as there are a lot of them and the area is very open. The achievement will unlock once you get out of the guarded areas of the level.