I found this to be an easier solution.
Load up the first level on easy mode, equip the silencer on both weapons and the tactical attachment on the scar. where you have to kill the first guy switch your bullets to the dart (tactical attachment) first, then go into camo and go to the guy, come out of camo shoot him then go back into camo. Then use the silenced pistol on his unconcious body, from where he was standing shoot the other two guys with the dart using the same method then go up to them and shoot their bodies with the silenced pistol. Repeat this method for the next two guards you encounter just after this small cutscene showing that dead nanosuit guy, aztek i think hes called. After shooting the 5th guys body with the silenced pistol the achievement should be yours, doing this my threat indicator did not move at all and i found this to be easier than the other ways mentioned.