Gears of War 2

I completed this last night with only 2 other people. We did this on the map UNDER HILL. There is a garage like place where you can open the door. It only has one way in and out with the door behind you closed. It has almost every weapon and grenade spawning somewhere on the map, and last but not least it has a shield that spawns. If you set up camp in this garage like area, there is a ammo box that constantly spawns just in front of you. Once you set up the shield to block the only entrance, the rest if fairly simple. There are side openings on the inside of this room, but no horde can get into them. Keep in mind that they can shoot into them though. We only died on level 40 a couple times and then on level 49 and 50 about 6 times each.

This achievement appears to be glitched as well. My team and I died on level 50, but I was separated from them and for some reason the achievement popped and it showed on my screen that the wave was completed. The other 2 guys on my team said they didn't get it at the same time. I was lagging very badly though towards the end, I could barely control Marcus and was lucky to get 3 or 4 kills per round.

It's a bit late to post my solution for this achievement, but frankly I'm kind of bored and I have nothing better to do. The first part of this guide will be a general Horde strategy while the second part will be my story of how I got this achievment.

This achievement requires you to complete all 50 waves on a Snowblind map (Under Hill, Fuel Depot, Grind Yard, Courtyard). How you go about doing this shouldn't matter. You can tackle all 50 levels on one map, 25 levels on one and the next 25 on another, etc. Also, these levels do not have to be completed in one session. How my friends and I did this was simple: 10 waves per night. If you didn't unlock he achievement for beating all 50 waves on any level, you can do so on any of the DLC maps. Also, difficulty doesn't matter so when going for the achievement, play on Casual. However you decide to do this, keep in mind that the basics of Horde mode still apply.
It's always a good idea to have a team of 5. Better yet, it's always a good idea to have a team of 5 people that know what they're doing. Playing with 4 other randoms will probably not net you this achievement. If you know people who know how to play Horde and are willing to help you out, get them in the party right away. It's all about communication and planning. Also, I suggest everyone play as a different character to avoid confusion. I find myself able to identify who I'm trying to help/who's helping me by their character.
Since Horde is about teamwork, it's a good idea to stick together by finding a "base" in a level. This will be an area where everyone can stay and attack just about everyone in the level without too much trouble, and without being attacked to easily. To make a base, Boomshields are mandatory (and absolutely necessary).
Boomshields will block off passageways and prevent enemies from reaching you. Drone varients can knock over shields, but the small enemies (Wretches, Tickers), Boomer varients, and those damn Bloodmounts are unable to knock them down. They will usually just sit next to the shield, allowing for easy kills. Not every map has a Boomshield spawn, so multiple shields will have to be picked up by the players after the Mauler rounds.
After a new wave starts, all loose weapons will be cleared. This means everyone has 10 seconds to replant the shields in their proper spots before they disappear. All ammo and weapon collecting should be done at the end of the previous round when only 1 enemy remains.

Now, for how I got this achievement:

Originally, I was part of a group of 5 and we were playing on Fuel Depot. After about 10 waves our fifth member quit playing but stayed in the match. Because of this, we actually had a pretty easy experience until he actually quit the match.
We made our based the hangar (spawn opposite of the "outside" spawn). We blocked off the huge entrance with two shields when we were able to. Grenades spawned just outside and we also had easy access to Longshot/Torque and an ammo box. Hammer of Dawn/Mortar was a bit out of the way to get, and they weren't really as useful after the first 10 rounds.
The strategy for the map was simple: Kill everyone outside and stay inside (except to grab weapons). The Boomshields were mostly up to keep the Boomer varients outside. The only bad thing is that all the Drones, Wretches, and Bloodmounts were able to get inside of the hangar. Eliminating the Bloodmounts on the 10th waves were always a priority. We would use enemy Mulchers when we could as well as frags and inks.
Now, I would still recommend having 5 people even though we beat it with 4. The team should be split up into 2 groups. The first group is the main force of four players. They will be defending the entrance and killing everyone. The other "group" consists of just one player. He will stay at the very back of spawn. Normally, this person won't be attacking much, but if they want to snag a boomshield and a longshot, then that's fine. This will allow them to help out that people in the frontline.
The reason this guy is staying at spawn is to prevent enemies from spawning inside the hangar. Since it is a regular spawn point, enemies will spawn there. We found this out the hard way when we were suddenly being attacked by Mulchers from behind. The support guy will work with mostly longer-ranged weapons to act as support. The boomshielf this person has is just in case someone breaks into the hangar. The person can then use it to turtle in a corner.

Now, things that shouldn't be done during this experience are simple. No one should leave the hangar to grabs weapons or ammo in the middle of a wave. The front of the hangar is going to be hectic. If someone is out of ammo and if things seem calm enough, they can get ammo as long as someone else is able to cover them. If they're trying to grab the weapon on the other side of the map, they will have to wait until there is only 1 enemy left and they should have someone with them just in case.
Also, if the hangar is breached because some players died, the remaining player should fall back to the support guy. If everyone on the frontline dies, its unlikely that the support guy will be able to finish the round on his own unless there are 5 or less enemies. Even then, it really depends on the enemies and how close they are.
Also, I didn't find the Longshot/Torque spawn to be a great spot to hide. It's rather hard to kill enemies from in here, but what's worse is that the Kantus will be able to kill you from an Ink grenade. The poison will go through the walls and slowly/quickly kill you depending on what wave it is.

This is the basic strategy that my friends and I used when going for this achievement. It isn't a strategy that ensures victory without dying so it may be a bit harder to accomplish if you're playing with people unfamiliar with Horde mode. Also, since it practically depends on a 5 person squad it isn't the best strategy to follow if one can't play on Live.

Just got this achievement on Underhill. I noviced out and put it on casual since it was just me. I used the shield to block the doorway in the garage, then laid in wait for the horde. Of course, make sure the Garage Door is CLOSED. Once you close it, it won't reopen by itself. Only thing about this method is that if you die, you not only lose the shield, but it doesn't respawn until you pass that wave.

TIPS USING THIS METHOD IF SHIELD IS NOT AVAILABLE:
Here's a way to survive in case your shield is no where to be found.

I found that if you walk backwards from the doorway, and back up all the way to the garage door (behind the pickup truck), enemies that file in to get you won't survive the onslaught of the lancer. This even works for the bloodmounts. Just be careful of Grinders that fire in at you from the sides. This caught me a couple times.

BloodMounts: Shoot the "horse" not the rider. When the horse is dead, then focus on the rider.

Maulers: Shoot their feet if they have their shield up. Then Grab that shield when they are dead.

Grinders: Take cover behind the truck for these guys, and use your blindfire. It WORKS!

Everyone Else: Fire at will!

Hope this helps someone. This method took me a bunch of tries, but it worked and I got through by myself. If I had someone to help/revive me, it would have gone by MUCH faster.

If you have tried everyone elses techniques and still cannot complete 1-50, try the Grind Yard, but instead of sitting in the hut in the middle, use one of the "outer huts" along the walkway that goes around about half of the map. This is the little building that has an ammo box next to them. This place becomes easy to hold down once you have a few shields made easier by the fact that you can see a great deal of the map from either of the buildings and the ammo box spawns there so there is no need to use just a Boomshot or a Mulcher. Also as a side note, if you hold down the upper corner buildings like I suggest, the chokepoint on the ramp should have a shield on the bottom of the ramp so that nothing can leap over the wall as enemies will do if they can get to the top of the ramp and the choke point towards the middle ramp that should be used is paralell with the boxes right outside of the door. Two shields will cover this. Three people should have no problem with this solution, any more and you may want to consider holding down the entire walkway so there is no fighting over ammo.