The cave where the Shadow Lake Horde are is on the north edge of the map north of Marion Forks. When they come out at night they travel between the cave and the mass grave site behind the buildings on the west side of Marion Forks. I think you can easily take them out anywhere along the road or at the mass grave, there are bushes along the road and all around the mass grave.
When I went there I didn’t know where they were going to be, as it turned out they were right across from a big bushy area down by the shore. The only thing I had to watch for was Freakers that weren’t part of the Horde roaming around behind me. I ended up using 6 molotov cocktails and 3 grenades, when you are taking these hordes out keep in mind that the noise from the whatever you use is going to attract other Freakers that are in the area. This mission pays 2000XP, 1200 credits, 1550 trust points and the Horde Killer story line will be updated.
Thanks for watching, God Bless and happy gaming.