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

Before attempting this achievement you will want to have at least three Runes available. With the required Runes on hand, open the character menu by pressing [ Register or Signin to view external links. ] and scroll over the the "Powers" tab. Now scroll down to the bottom block of skills and unlock the "Reflexes" skill (1 Rune), followed by the "Superior Deflection" skill (2 Runes). These skills will deflect any projectile to the closest enemy while you are parrying in combat.

All that's left to do is to find an enemy guard who wields a pistol. Once you've tracked one down, engage in open combat, distance yourself quite a bit so that they are forced to use their firearm, and tap [ Register or Signin to view external links. ] as they shoot their pistol to block and deflect the bullet. Do this twice (or thrice depending on how much health they have) and it will kill the enemy, thus unlocking the achievement.

NOTE: This achievement cannot be obtained on a "No Powers" playthrough since the "Powers" tab in the character menu will be unavailable, therefore disallowing you to choose the skills necessary for proper setup.