This is very simple. The room to do this in is the Solar Engine in the Tower of Cogs. The first time you enter it you will not be able to get this achievement since there are no corpses present. After you get and charge the battery found in here, unlock the Grasp, and then enter Sebastian's nearby study (about the midpoint of the game), you'll get hold of a gear which you can bring back into this room and place in the Solar Engine. This will cause the machine in the room to rearrange and set you on the task of powering the Solar Engine. Before you do that, however, you may have noted that when the machine rearranged a body fell down from the ceiling. Grab it with your telekenesis power and toss it into the shaft (opposite of where you entered the room) to unlock the achievement.