Everything TheGearGuardian stated is true. I just want to give a quick note on doing this easy if you have any issues under pressure. After you get back from getting the backpack you have a second mission. Once your friend on the com helps with the sliding verticle doors .... You can see a mounted gun. If you interact with it he will advise that you have inbound zombies. If you head back under the door(behind you that you came through), the zombie will get on the ground to crawl under after you. This makes for easy aiming and achievement w/o trying to react quickly.