This guide is about the Greywood Outpost Attack - but it can be applied to any other. In this guide it'll be explained and shown on a video what you should do if the instance you're in has a hard time holding the door and surviving in general.
The most important thing to realize when Greywood OA starts is the strength of the team in your lobby. It can easily be checked just by looking at their weapons. If you have melee people with anything that's not a Sledgehammer or Longsword you know that you're gonna have a bad time because they'll be rather useless. This is just so you can prepare yourself.
If your team is unable to keep the zombies off the door and it's constantly getting hit - the best and pretty much only thing to do is to start kneecapping the zombies that spawn further away from the door (Why should you ask? Just in case so that people in your lobby don't instantly kill them after you kneecap them). However, do not forget to tell them to not engage kneecapped zombies and instead to focus on non-cralwers.
So move away from the door as far as you can where the zombies are spawning and start kneecapping them.
After you stacked a bunch of zombies and let them "swim", go back to the door and help with clearing everything that's still running around. If you still find it too hard (or too easy), you can go and kneecap more zombies or just kill some crawlers.
Keep in mind that the crawlers will eventually come too close, and people might kill them on accident or so they don't get hit. So you'll have to re-do the whole kneecapping process a few times during the GW OA.
If everyone who's in a lobby that doesn't have a bunch of overgeared players starts doing this, we'll have a way higher chance of winning Greywood attacks, because mostly when we lose Greywood OAs it's by a margin.
Kneecapping Process Shown on a Video
The reason why the video is a bit closer to the door is because of the time left on the OA - I decided to make a video in the last moment. But in most cases you can go as far as where the cars are and you'll have enough zombies spawning for kneecapping.