Must be done on at least Normal.
Start the mission from the beginning and fight the first couple of waves. Shortly after starting, you will push through the base and go round a corner and down a small hill, coming to a gated room/area. In this area, on the left of the room are some weapons. Pick up the target locator. Head out, and you'll see a Wraith. Wait a few seconds and another will come into view. Wait for these two to come within close range of each other, then target the area with the Target Locator. Boom!