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This is the same guide I posted on True Achievements
To unlock this achievement you must survive the first three rounds without letting any zombies through the barrier. They can remove the boards just do not let them enter through the barrier as if one zombie hops over the barrier it will void the achievement.
This Achievement is much easier to do with multiple people but here is the setup I used to get this achievement and I thought it worked very well. In this guide I will include a two versions. One for people who have Mega Gobblegums to use and the other one for people who don't have them or are to lazy to risk using them.

Mega Gobblegum Guide
Setup - For my gobble gums I used a setup that gave me something useful in each spot

Shopping Free - Use to hit gobblegum machine. If you do not get your desired gobblegum go ahead and buy RK5 and the Shieva while you wait for machine to reset so you can hit it again.
Extra Credit- Same use as shopping free although you will only get one additional spin on the gobblegum machine
Perkaholic - I equipped this with the intentions of using the Widows Wine knife so I could one knife every zombie when it got to the barricade
Round Robbin - I used this to skip a round and get an extra 1500 points to hit for another gobblegum
Board to death - This is the least useful gobblegum in the pack but I had it on just so I could run around and kill zombies without much thought

Strategy - When you spawn in go straight for the gobble gum machine. When you hit the machine your looking to get Extra Credit, Shopping Free, or Round Robbin. In my game I got Extra Credit first and I popped it so I could buy the Rk5 and Shieva as well as hit for another gobblegum. I got Round Robbin for my second spin. So I popped it and to skip round 1. The next round I went and I got a Perkaholic for my free gumball and when I payed for the next one I got Board to Death I did not pickup the gum and instead I just used my knife to end the round. On round 3 I hit for the gobblegum machine and got Shopping Free which I used to hit the for another Round Robbin. I got it on my second spin immediately popped it to end the round. My Achievement popped at the start of round 3.

Side Note - If your using Mega gobblegum don't forget to close app to keep them :)

Classic Gobblegum guide
Setup - These are the gobblegums I thought of using and I think this is the best setup if you are only using classics
In Plain Sight - I would have this on and pop it if a zombie got to close to the barrier as in my experience the zombies will completely stop moving outside the barricade giving you enough time
Sword Flay - This gumball will essentially give you a 1 hit knife kill for 5 minutes
Always Done Swiftly - I would try and use this to aim in faster for headshots with the Shieva
Armamental Accomplishment - This gumball will allow you to knife faster as well as decrease the time it takes to reload your gun.
Stock Option - This can be used to give you more time to shoot the zombies as you will not need to reload while this gumball is active.

When you spawn in I would immediately grab the RK5, prone next to the Mule Kick machine to get the extra 100 points and then hit the Gobblegum machine and get one of the gums in my pack. I would recommend running around the spawn room and kill the zombies you see in the distance. Once you have enough points buy the Shieva and continue running around and killing the zombies while using the gobblegums to aid you in your quest

I know this is a very simple Achievement but some people can get overwhelmed with Nacht Der Untoten being so slow in the early rounds cause it is very easily to get distracted while you wait for the zombies to lurk towards the window. Happy Hunting :)