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This is pretty simple and is easy way to take top spot on leaderboard as long as you have patience.
Go down the first lift to the middle floor. Where you would normally open the barricade on the stairs you need to run and strafe jump from top of either staircase aiming to jump over the barricade which you will eventually get behind.
Once behind the barricade feel free to leave Xbox on and the zombies cannot get u and they will eventually die. But make sure you DON'T open any other doors once over barricade..