In "Under the Radar", right when you get your car back from the mechanic crew don't leave the base just yet. You will face a straight road where Zombies spawn infinitely. Just wait until a couple of them gets on the road, crush them, come back and wait again. Do not worry about the guards, they seem to never be able to kill the Zombies, but don't get too close or Alyx will kill them.
It can be a bit boring (mostly because the Zombies walk so slowly) but you'll easily get to 20.