How to Acquire
Emily can use Domino to link the fates of four enemies together, making it easier, but must still find a way to kill three enemies (two normal ones, plus a third linked to another three) at the same time. She can keep up to four enemies immobilised in a location by using her Mesmerize supernatural ability. She can also combine Mesmerize and Domino, mesmerizing up to seven enemies, and kill at the enemy that is both linked and mesmerized and two of the other mesmerized guards. If this is done quickly enough, the achievement will be unlocked. Note that it takes time for effects to travel through a Domino link, reducing the effectiveness of this strategy.
There are various ways to kill large groups of enemies, including grenades and whale oil tanks. Finding and activating an alarm can draw these enemies together, or it can be done manually. However, by far the easiest way to earn this achievement is to knock out six enemies, pile up their unconscious bodies and detonate a grenade, spring razor or whale oil tank near this pile, killing them all.