I recently found a guide to obtain the Zombie Genocide Master achievement.

It’s basically summarised as:
1. Grab the Driving Magazine (This is found at the soda fountain in Bennie Jack’s BBQ Shack in Americana Casino)

2. Use the SUV to kill zombies outside from Silver Strip down to Platinum Strip.  You will have to complete the Story Cases throughout though to ensure you get the extra time required from Overtime Mode to hit that magic 72,000 number.

Milestones:
5,000 Kills - 25/09  14:00
10,000 Kills - 25/09  22:30
20,000 Kills - 26/09  10:00
30,000 Kills - 27/09  00:00 (Midnight)
40,000 Kills - 27/09  12:30
53,596 Kills - 28/09  04:00
72,000 Kills - 29/09  05:00

3. (Optional)  Grab the Biker magazine in Case 3-2 (On a sofa in the Slot Ranch Casino mission area) if you want to also kill zombies using a chainsaw-enhanced bike.

(Source: PowerPyx Productions)

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