As requested by Sol76 i play tested my solution and it worked again.
Nevertheless i present an exact description how i pulled it off.
I did it right at the start of the game. I looted the first box, grabbed the assault rifle (with suppressor) and went northwards.
If you wanna farm the achievement i suggest that area anyways.
I roamed around looking for 2 men groups patrolling the area. They are shown as red clouds on your radar (play in arcade mode).
-If you spot groups, observe them with your scope and mark them.
-If they are marked, the red cloud will disappear from the radar so you can tell how many they are.
-Avoid groups of 3.
-Next thing you do is to circle them. Go close on them from behind and line them up.
-First guy will get a headshot and the next one will eat the whole clip. Shoot until they drop.
Repeat that 6 times and there you have it.
They can start to spot you, 2 guys saw me. As long as they go down fast enough, you´re fine.
It seems that close quarter kills maybe don´t count. I suggest to use an assault rifle.