Have the bucket on for almost the entire time. The timer starts whenever you hear the portals. So just hide under the bucket and only close the door when the security zombie hasn’t come in a while or when you just feel something is... off. Then you will win. It does take some luck as one of the zombies is not guaranteed to be fooled by the bucket. You have to survive for 7 to 8 minutes. Each hour takes 70 seconds and you have to survive for 6 hours. GOOD LUCK.