im addicted to the zombies too, only got it on PC.
There is a good tactic where you do open upstairs and leave the lower door locked.
If you have 4 people it is really easy to hold to 20+
Tacs are:
Dont open lower door
On lvl 5 try to keep last zombie alive (this will give you more time to get weapons and clr the path) just keep rebuilding with one at window.
Clr both barriers on stairs and keep trying for weapons (mg/rays best for holding bottom of stairs later)
When lvl 6 starts have 1 downstairs in helproom on windows/wall and 3 upstairs (have 1 standing by stairs leading up from original room)
On lvl 9 all fall back into help room
Then you would have 2 at bottom of stairs, 1 on wall and 1 covering windows. The important bit for the people at the bottom of the stairs is to stay center directly behind one another and do not move.
The one infront would crouch and 1 would stand directly behind him. This is the really important bit as you ideally want the zombies coming directly centre down the stairs. If you have people closer to the zombies, standing left or right they will drift off center making it a lot harder to headshot. Using the box and standing by it again can make the zombies drift off towards that person.
You basically are using the front guy as bait for the zombies, they will all run directly for him if he doesnt move. if anyone is closer on box or left or right, they will go towards them. They always go for the closest person.
The person standing on wall would help both the windows if getting overun or the stairs when the are reloading but his 1st job is always rebuilding wall.
The player on the windows just has to scoot between two windows keeping rebuilt.
If you have points to spend try to save the last zombie each round, either holding one at window/wall or running the last one around. This again gives loads of chances for everyone to get rays etc.