Great solution by Xv iLeGenDzZ vX
I used a slightly different method. There are 3 cars in the maintenance tunnels that you can use to maximize zombie deaths for this achievement. Here's my method:
(1) definitely grab some OJ and the horror 2 book as mentioned by Xv iLeGenDzZ vX
(2) head to the maintenance tunnel entrance which is accessed by going through the park
(3) get in the red convertible and enter the tunnels
(4) take an immediate right, then a left, then a right, then a left which will put you at the white car
(5) get in the white car and follow the attached map: right, left, right, left again which puts you at the white van
(6) get in the white van and drive the loop
(7) head up the ramp and get in the red convertible again
I averaged about 1200 kills per loop with this method and managed to have lots of fun despite the repetition. Your zombie mileage may vary
Helpful map here:
http://imgur.com/sYZ93R2I marked the maintenance tunnel key on the map as well ... if you want to save during your run at this achievement make sure you get the key then head up the stairs by the white car to put you back in Paradise Plaza right by the bathrooms for a quick save. When you re-load ... head down the stairs and start with the white car.