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This achievement can be earned at any time during your playthrough and can be done wither either Corvo or Emily.

The earliest and easiest opportunity to work on this is during Mission 1, A Long Day in Dunwall. Before leaving Emily's safe room, make sure to loot the pistol and the extra bullets on the workstation near the tower's exit. Once you reach the city streets, and after learning about Drop Assassinations and taking out the first guard in the courtyard below, climb the nearby stairs and hop on the ledge to get a full view of the street. You should see four guards speaking.

Make a manual save here (you can revert back to this save if you mess up) and drop down to the street. Now equip the pistol, sprint towards the guards, and tap [ Register or Signin to view external links. ] to slide. As you are sliding, adjust your reticle as necessary and try to fire a single bullet at one of the guard's heads. If you're lucky, you should score a headshot and unlock the achievement, though don't feel defeated if it takes a few tries.

NOTE: If you are having trouble getting the achievement on the first mission, you can try again later after using Runes to unlock the "Reflexes" skill and its upgrade, "Focused Slide."