You either need to be close to the walker to get credit or score the killing shot. By close, I mean VERY close. In order to get credit for it, I stand behind the tiny hill near the entrance to that area (closest to the garage on the wall where you come out of the tunnel). It's the hill straight in front as you enter that area (sorry I don't know the name) and it usually has 3 Dregs standing on/around it. If you hang out behind that hill you have cover (mostly) from the walker, some from the ads, and you get credit regardless of who kills it. But if you are more than a few seconds run away, and it dies, you won't get credit. You need to almost be on top of it (which is totally stupid).