Go from where you go to the elevator and theres a fence to the right (dont go in elevator)and get say a semi auto rifle if you have it and shoot zombies from there you cant get attacked cause there in the parking lot and if you have like 100 shts and you dont use the semi auto rifle in the missions then its almost free you just need to buy the gun so its not possiblr to get hurt cause the zombies cant come up behind you if there arent any.