This achievement can be obtained in Redfall Commons. Head north of the Autumn Trail Road map marker, or southwest from the Golden View Mobile Community map marker in Basswood until you arrive at the Hilltop Grave Site. Once there you will see a flare on the ground - equip any flare gun and fire it off. Shortly after a unique Shroud named The Wailing Shadow will spawn. The achievement will unlock immediately after the named enemy is killed.