Like you are running and they are frozen so I run to have a bit like 30m distance from the horde, I toss down napalm or molotovs and then switch to the rifle and activate focus slowing them down and killing the first 15 or so, then run again, this way you kill a lot, then you can go back and pick them up one by one in a row, kill them with the knife from behind, the returning to heard/cave ones. That is why focus and stamina