Clear the debris to the upstairs area and head upstairs. When you first get to the top, you will see an area where you can resupply grenades. Head directly in front of the grenades and have your back towards the grenades. You are now in a great camp-out location. Since you are on the top floor you have access to the flamethrower, the trench gun, the BAR, and grenades.
After reaching 3,000 points, go upstairs in the zombie factory place. While upstairs, if you used the main steps on the front start area, turn around. Go around the steps and go back to the back of the room near the sandbags until you see a vanity or dresser object behind a pillar. Go up to it and open it to get a flamethrower. Go around the pillar over to where the steps are located. Turn left and to find a room and a wall in a protected corner. Go in the corner and use the grenades behind you and your flamethrower to kill.
Unlock the "help" room. Once there, unplug your charger and plug it back in. To exit, do the same thing as you did to get in. Note: You are invisible to the zombies the whole time. If you are having trouble killing them, shoot up and you will get a zombie or two.
To obtain a free Wunderwaffe DG-2, open all areas but do not get the Perk-A-Colas. After getting them all open, you will receive a shovel when you use the knife. Search for a mound of dirt. Tap the "Dig" action button to receive the Wunderwaffe.