Pretty self explained, complete all 50 waves of horde mode on one of the five maps from the forces of nature map pack, which include Ravendown, Cove, Aftermath, Artillery and Jacinto.
i did this with one friend on Ravendown, on casual, with infinite ammo and super reload. What will also make this go faster is setting up base near the tree in the garden area. You can build one dummy, and two command centers, after you've built those and have enough for the other command post (starting base post) you will be able to build a third Command center, and use three a wave, giving you alot of money (most the time) and taking out about half the wave if you're luckey. Also don't be afraid to use it during the boss battle, I had the brumak boss wave, killed the brumak as soon as it spawned with the command center and killed a total of three in that one round. After you beat the 50th wave you're achievement will pop.
Side note, if you don't have the Super reload Mutator or the infinite ammo Mutator, heres how to unlock them.
Super Reload = You have to bronze the Master at arms medal, which is getting 100 kills with the Lancer, Retro Lancer, Hammerburst, Sawed-off, and the Gnasher. Easy way of doing this is going into a private King of the hill game with full bots and just run around and kill.
Infinite Ammo = You need to get the Combat Engineer ribbon 100 times. To get the ribbon, go into horde and during every round, build, repair or upgrade five fortifications, doesn't have to be five different ones, you can get 1 ribbon a round but an easy way of doing this is to have a friend play with you, and build a turret have the friend get on and shoot till you've reloaded the turret five times, its very cheap and can be done in like 3 seconds. I'm sure there are different ways of doing this, this is just how I did it.
Hope this guide helped, any of these tips can be used for anything horde related.
Thanks to, DEV1LWE4RSPRADA for pointing out. If you clear the update, by going into your cashe and clearing it, then loading up gears offline, you can use the turret trick for the infinite ammo mutator but get more than one ribbon a round.