Head to the Infected Factory location (the same , but with beta and Halloween-themed decorations, accessible from the main menu; you need to switch the locations to event locations in the top-right corner), and shoot the zombies spawning throughout the location that will run toward you on their own. Be careful, the location still has in his infected version. Bring plenty of loaded magazines; there will be a lot of zombies, and reloading your ammunition will be problematic. Try to shoot them in the head; a few hits should be enough.
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Skier