Ensuring you leave one of the buildings active in Northern Wall, once you have a scythe go there with a few units (enough to clear out the wandering units outside the building), then send your units away (just order them to attack in the opposite direction of the house and ensure there's enough distance for them to travel without stopping so they won't aggro) and stand in front of the door so all who spawn are in-front of you. Swing at them with your scythe until you kill 3 (NOTE: you will only need 3, the details which say 6 are wrong) in one blow - this may take a few attempts.