Here's a map of the underground with a key. Sort of.
My path was going to the top-left, getting the DV, then following the blue line in the DV, picking up the white car, and lapping the bottom-right tunnel (in the white car), and following the blue line back to the Meat processing plant.
When I did this, I averaged at least 1000 kills doing ^^this^^ each time. Hope this helps you a little bit.