(adsbygoogle = window.adsbygoogle || []).push({}); Ok, so I actually got both of these trophies without the help of any barriers.
You will need:
3-4 people who will actually cooperate

Beginning strategy:
Play normally. After round 3 open up the stairs (back gate must be closed at all times)

Everyone will need:
-Bouncing betties
-Thompson
-Buy both of these first in that order. Then you can spend extra points on the random box

Everyone will camp in the area on the opposite side of the gate. It's after you come down the stairs, and go up the other stairs. That's the same area with the M1A1 Carbine off the wall, as well as where the random box sometimes spawns.
The exact spots are the following:

Stairs (1-2 people)
This is where most zombies will come. Whoever has the best guns/most ammo needs to guard here. You should stay at the bottom of the stairs and only go to the top when you need to. This way you have more running space so you're not overwhelmed as easily.

Front Window (1 person)
This window is the one that faces the flogger. The zombies that come to here walk on that balcony that you can shoot at from downstairs. You will have quite a few zombies here.

Back Window (1 person)
This is the one directly accross from the upstairs zipline area. You shouldn't encounter many zombies here.

Betty Placement
You get 2 every round. And if you play well, none of them will go off. For windows, the primary betty should be about a foot away from the window. This way, it only goes off if the zombies break in. The rest of your betties should be placed around, but spaced far apart so multiple don't go off at once. For the stair guard(s), place them behind where you stand, so if you're in trouble/reloading, fall back and boom.

Dog Rounds
You should still camp in corners in your upstairs area. All those extra betties really come in handy. Good corners would be in front of the zipline, at the back window, and corners inside the room with the front window

But wait, how will we get weapons and perks?
No crawlers. They are really hard to make in the positions you're at, and even if you have them, they'll hit betties. So when there aren't many left, have people hold them at both windows. Usually, you'll be able to do it from the front window. Just have the person always hold that zombie. And don't just hold square/X, but hold until the board is placed, then let go and do it again. If you just hold, then you will stop repairing and the zombie will break in. Also keep this in mind when checking the scoreboard, as if you're holding any button and bring up the scoreboard, that hold will dissappear if you get rid of the scoreboard. Like if you're firing a machine gun, holding the trigger. You bring up the scoreboard real quick and get rid of it. Your gun will stop firing for a second.
So anyways, while one guy is holding a zombie at a window, everyone else can explore the map and do their deeds

Your best friend
The multiple gun glitch. In the higher rounds, we ran out of ammo very quickly, and needed all the guns we had. The multiple gun glitch is pretty common, but a quick rundown for those who still don't know it:
1. This only works when the random box is in the fishing hut or doctor's quarters.
2. Open the box
3. Wait for the final weapon to appear (the one you get when you trade guns)
4. Buy the perk machine
5. As soon as you start buying, don't just stand there. Like as soon as you start the action, run to the box
6. Right when you're at the box, press square/X to take your weapon
If done right, you will have switched the perk bottle with a gun. Make sure you have enough points for the box and perk

Special Conditions

Stairs person/people need(s) help
Whoever guards the back window can stand in the corner by the zipline. From here, you have a perfect shot of your window, and the top of the stairs. You can really lay down supporting fire

You only have 2 people available
One person gets stairs, the other must find a spot where they can see both windows. Front window needs more attention than back window

Somebody gets the Ray Gun
NEVER use the Ray Gun if you're guarding the stairs. You will get blown up. Even if you have Juggernog, any zombie running at you will be perfect for you to blow yourself up. Whoever has the Ray Gun should be in the zipline corner. Perfect spot to assist the stairs person, and will be able to kill the zombies at your window before they even remove boards

Low ammo
Even if you don't have the best guns, whoever has the most ammo in a low ammo situation needs to get stairs. Moderate amount of ammo gets front window. Low ammo gets back window.

Stair person is downed
In most situations, both window people will be able to assist. But if not, only the back window person.

Front window person is downed
Back window must save them

Back window is downed
You're most likely screwed, as you must be pretty bad to be downed there. Unless you have low resistance, it will be hard to get this person.

You just can't stop them
Make sure the zipline door is open. Stay on here as long as possible and just jump down. Run into the doctor's quarters. Now if you have multiple people down there (unlikely) go inside, turn on the electric barrier, and guard the windows. If you're by yourself, it will be harder to guard the windows, but it's possible. You will need a machine gun though. After the electricity wears off, you can either risk running past the zombies, or go to the zipline. From here, just improvise. Never shoot at zombies while moving, just keep running until you can pin them down.

This was tough