How to Acquire
A very easy way of achieving this in a high chaos playthrough is to enter the building to the right of the Boyle Estate main gates, in the mission Lady Boyle's Last Party. At the bottom of the stairs, there are six weepers in close proximity to each other. Run towards them until they are on high alert, then Bend Time, knock out five of them (leaving one conscious), and unfreeze time. The remaining weeper's area attack will kill the five unconscious weepers, and the achievement will unlock.
An even easier method can be executed during the mission House of Pleasure. In the Distillery District, knock out five thugs, and line them up on the barrels behind the small wall. Get behind a thug and attempt to knock him out. As he is hostile now, run to the barrels, and he will burn both Corvo and his allies.
Another easy method is to knock any guard unconscious and carry his body until being sighted by tallboys. When they shoot their incendiary bolts at Corvo they will kill their ally. However, this must be done multiple times to unlock the achievement, and the tallboys can do considerable damage to Corvo.
The enemies do not have to be unlocked at the same time, or on the same mission, and not all the enemies have to be of the same class, provided they are killed by one of the same.