***Please read the whole thing before giving thumbs down. ****
The above advice is good, this is just how I did it and how I thought it was easier.
If you are wanting to get this or any horde achievements play social horde. Especially on the 5X, 10, 20X weekends. Lots more people play then. Granted you don't get to pick the map, but I found eventually I could get all the maps I needed this way.
Also during the matches, talk to your team mates. revive them if they're down. If you get killed during a wave, watch your team mates. If someone is sneaking up behind them, tell them. If they are running up to a corner and a grinder is right around the corner, let 'em know.
In other words play as a team. I made several friends like this. Then if you or your new friends need a few waves in this or another map, do them.
I had 1 map where I needed 30 waves out of 50 or so. Not in sequential order either. ( playing social, this happens. Like the host leaves kicking everyone out, or you show up at wave 14 or whatever)
I found myself in a match with 3 other people who needed the same waves in a certain map like I did. So I started a social lobby, invited each of them. With 4 of us every time we started we ended up starting at wave 1. It took about 15 minutes of starting and going back to the lobby until we got the right map. Then we did all 50 waves. A couple of the guys left after they got the waves they needed ( like they said they would ) and others joined after to help out. That's why I like social. Some leave, others join.
Sometimes it sucks if the host leaves the before the end, but you can eventually get them later.
And for those who don't know, the way to set the difficulty in social is: what ever you playing last in SP is the difficulty you are on IF YOU ARE THE HOST. So if you just finished the campaign in insane and went right to playing social horde, be ready for horde on insane.
If you want to change it, start a new SP game or continue a saved game on the difficulty you want. It will stay there until you play SP on a different difficulty. You only have to play it for 30 seconds or so.
Also 1 more tip. Sometimes maps like this one, 50 waves is a lot. Maybe you don't have the time to do the whole thing. If you have made a few friends, ask them to do 10 waves at a time. Do 10 now, 10 tomorrow and so forth. You'll find that some people don't have 2+ hours to do 50 waves now. But they might have 45 minutes to do some now and more later.
Custom is good for this. you can set the difficulty, map and starting wave. I did this a few times, but the rest I got in social.
**************Good luck People. **********************