You get this achievement by performing a melee attack (
) on unsuspecting enemies. There's no special stalk button you have to press - just walk up behind an enemy that does not see you and press
This needs to be done 30 times. When you fail a mission in the game and need to restart at the last checkpoint (which, I'm not proud to say, but happened to me plenty of times) the melee attacks you earned before you died carry over. This made the achievement easily attainable early in the game.
I haven't confirmed if you could just do this to pedestrians in the game instead of actual enemies, but there are plenty of security guards in the game that you should not have to bother farming this.