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Go to The Crossroads or somewhere similar with lots of zombies/enemies and cars. Throw cars at them or beat them with the cars. Rinse and repeat.
There are other topics too, and one pointed out that using bombs counts for this. You can go to one of the rooms in Iron Canopy that has bombs and lots of little spiders and use that method instead. It's much quicker, but you can't fast travel there like you can The Crossroads.