I recommend for this achievement the easy difficulty. There are 5 brothers on the map. Don't kill any of these brothers to obtain this achievement. Here are the 5 locations on the map:
I show you in my video a very fast way to obtain this achievement in 6 minutes. Equip some bandages and supression ammo for any weapon you want and follow the video.
In the video I run straight to the only primary target and avoid all enemies in the village. At the end I kill only 7 enemies in the bunker area and run without killing an enemy to the end of the mission.