Go to the parking lot in Leisure Park and get in the convertible. Drive into the maintenance tunnels and grab the maintenance key in the "T" shaped dead end just ahead of where you went in. Get back in the car and drive over to the exit under Paradise Plaza. Switch to a white sedan there then drive all the way over to the exit under Seon's Supermarket. Kill as many zombies in that location as desired. Then, switch to the delivery truck there and drive once more around the maintenance tunnels until you reach the exit to the parking lot. Drive out and switch to the convertible and repeat everything except the maintenance key part. Try to choose an efficient route and stick to it. It will take a while but you will be able to get it if you begin doing it at the start of 72 Hours mode. It would be a good idea to stock up on wine or orange juice, have at least one nice melee weapon and the two Horror Novels to get the most out of the PP you earn from killing the zombies.