I am posting this as a reference guide for anyone who may be having trouble or is just new to zombies in general. You just need to get to round 20 you do not have to beat round 20 and you cannot open anything. Doing this solo is easiest. Also, you can set the difficulty to casual.
Step 1 go over and knife one of the ropes holding the banners to activate the challenges which will end up getting you the Z-Harmony once you complete 3 challenges. If any of your 3 challenges require you to open anything up simply restart the game and try for different challenges. After each challenge go up to the podium and claim your reward to start the next challenge. I got knife zombies, headshot kills, and special weapon kills.
Step 2 always try to have 4500 points on you that way you can walk over and get ammo off the wall. Even though the Z-Harmony is a modified version you can still buy ammo for it from off the wall.
Step 3 train your zombies. For anyone new to this watch in my video how in the later rounds I take paths around the arena and get all the zombies lined up to follow. This will help with points when you shoot because bullets will penetrate as well as buy you time to kill them off.
Step 4 once tigers come always get rid of them as quick as possible. Shouldn’t be hard to do but you want to get rid of them quick because they can ruin your trains.
Step 5 when the armored guards come don’t stand in one spot too long, unload some bullets on them and then move so they can’t hit you or harm you. They will take more shots then regular zombies so just keep moving and be patient.
Step 6 when the daddy long leg creature comes make sure to shoot the humps on his back. He can shoot his tongue out so do not stand still at any point. Shoot while running always!
Nukes are your best friends
If you get trapped in a spot because of fire don’t panic, you should have enough room to train in that area as I show in the video at times.
Avoid the center podium at all times unless an emergency its not going to help your trains and will break them.
Never buy anything but ammo or guns from the wall, opening any of the gates will void the achievement.
Get a feel for the fire timing and patterns to help you with your trains and not getting stuck or hurt.
If you go down don’t panic you have 3 revives, so don’t stress I promise you can obtain this achievement just follow my steps and video and you will have it in about 30 to 35 minutes or less.
If you need any help, please message me I am glad to help!