[ Register or Signin to view external links. ]

It is highly recommended to get this done in Mission 4, The Clockwork Mansion. Moreover, this achievement should be paired with a lethal High Chaos playthrough.

Once you arrive at the Clockwork Mansion, head into the parlor to the right of the foyer and pull the lever on the right-hand side of the room. This will spawn in a single Clockwork Solider and reconfigure the layout of the room, granting access to the rest of the mansion.

From here you will want to Blink/Far Reach onto balcony above the Soldier and wait until the machine is within Drop Assassination range. Drop onto the Solider, tap [ Register or Signin to view external links. ] to remove its head, then set yourself back into position. Attack the Solider one more time and when it's dismembered and dead, you can find a plaque among the debris. Do this twice more over the course of this mission and the achievement will unlock after picking up the third plaque.

NOTE: There are several other ways to eliminate Clockwork Soldiers and obtain their plaques: you can instantly kill them with a stun mine; you can shoot their heads off with a crossbow, then perform a drop assassination; you can attack their glowing limbs with your sword until they overheat and explode.