After the start when you go up the stairs when all the zombies come in and you've saved Jeff and Natalie.
Don't do the food court case!
After going in the park go to the North Plaza to get a sniper rifle, better yet get 2 rifles for a better chance and so you dont run out of ammo, the phsyco wont be in it because its too early on in the game.
Go back to the leisure park and kill the convicts from range, you should be able to kill them from anywhere in the park as long as they cant shoot you.
After they're done and dusted save the game, and do the cases or just kill zombies.