Gears of War 2

i found the best place to do this horde is in the building which has a mortar/mulcher on the roof, there are only 3 entrances and 2 are windows so they are easy to defend also if you can get a shield to block the door you'll find it alot easier, always try to do on private with friends as you can do it on casual whereas online the difficulty will be set to normal

I made it to wave 50 by going inside the building that has a mortar on the roof. You can hide behind the bar wich allows you to unlimitedly pick up bullets for your hand gun.

If you have a shield place it in the front door so no one can get in. If you have any bombs place it where the bar has openings so if they get in, they get blown away anyway.

Tip: Inviting one friend makes getting this archivement a piece of cake. (Make sure that you reached multiplayer xp level 50 to get this archivement). Good Luck!

Go to the building with the mulcher/mortar on top and don't go inside, instead go onto the roof where the mulcher/mortar spawns as there is only two staircases to cover. Make sure you make good use of the mulcher/mortar and in the later stages try to get some shield of the maulers to shield the staircase entrances. Make sure you plant the shield low enough so you don't get hit by grinder fire. You will need at least two people to do it with. One person covering each staircase. However with five people it should be achieved easily.

***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. **********************

I got this yesterday. We would wait and grab shields every round that we could get more and block off the main entrance to the main building. We would hide behind the counter.

One guy covered the left window, one on the right window, and three on the front who would also rotate. We did it on Hardcore and it took about 2 hours, so on Casual it should only take about an hour or so.

As with the other horde achievements, you don't need to do this in one go, ie. you can some waves on one session, and the other waves on another session, or however you want. However, tracking which waves you've done can be a pain (the in-game tracking system tells you how many you've done, but not which ones), so I made a printable checklist for all the achievements in pdf format.

Enjoy: http://www.mediafire.com/?u8nz9f8ia7ujbd1

I have found the best way to complete these fifty waves of horde, with one or two people. Go to the large building in the centre of the map that has the Grinder ontop of it, jump behind the bar where the boltok/gorgon pistol is and you should have two metal frame kind of things that have what looks like a washing machine/dish washer in them. Go to the back of this and set up two shields blocking yourself in. I found it works best with two people. You are near enough untouchable from anything here (maulers, bloodmounts ect.) Don't sit right at the back as a boomshot may get you, use the shields for cover to stop any ememies knocking them down. The only possible flaw is kantus throwing inks, make these your first priority to kill and you should be fine. For even more security get two more shields and block off the two openings to the bar at each side. Enjoy.