While in Act 1, Chapter: Ashes-Wrath, you must go up a set of stairs to get to the roof of the building. On the roof there will be Troikas. You must pick off the Locusts down on the landing zone. Near the Troika, directly in front of the steps where there is a Locust already running it, is a Hammer of Dawn hidden in the grass. You can use the Hammer of Dawn to kill all of the Locust on the ground with ease.