Achievement is to kill 12,345 zombies. Best done in Sandbox mode since you'll have unlimited time.
If you wish to bowl over zombies, earn enough money for the SUV and just keep running down zombies.
Or grab a chainsaw and modify the bike in the trailer by the Yucatan Casino.
Drive up and down the Strip killing zombies as you go. You'll get the achievement when you reach 12,345 zombies.
Note: Funny reference huh? It's pretty awesome.