Gears of War 2

Horde mode is good fun, isn't it?

This achievement is all about completing all 50 waves in Horde on any difficulty and on any map. First thing to note: You don't have to do all of the waves in order, or in a row. I got this achievement by first doing waves 28-50, then 1-9, then 10-27, all of which were on different maps and varying difficulties.
If you're going for the achievement and not some super-high score, put the difficulty on Casual and invite whoever you can. No matter what difficulty you are on, it's always a good idea to roll in with a full 5 person party.
Enemies will increase in their numbers the further along in Horde you are and they will also get stat boosts. In the earlier waves, enemies will be like they normally are, but later on they will get beefed up. Their health, accuracy, and damage will increase from x1, to x2, to x2.5, to x3.

Horde mode is all about the teamwork, so keep communicating with one another and if you can revive someone: do so. Each player is allowed to get downed twice before they our done for the round. Also, no one should be running around doing their own thing. Either everyone should find a location in a level to make base, or split up into two teams and try things like that.
I think to visually help out your teammates, everyone should play as a different person. It won't make much of a difference, but it can help out with communications since you'll know who is by you based off their in-game character.

Now, when it comes to weapons some are better than the others. When starting out, the Lancer is a decent weapon. It can dispose of most enemies quickly and it has that lovely chainsaw. However, I think the Hammerburst makes for a better weapon overall. It has a zoom, deals more damage, and enemies tend to drop this weapon more, making ammo plentiful. The chainsaw is a bit unnecessary in Horde, but it does work wonders against the Sire.
When it comes to the other weapons, though, some people will always want certain weapons more than others, which is fine. As a rule, though, make sure there aren't many enemies left when going out for ammo and new weapons. Breaking formation makes the teams' weakpoints to be exposed. Also, it's good to make sure everyone is back in formation before killing off all of the enemies.
When a new round starts, you have 10-15 seconds to grab the leftover weapons before they disappear and before the new enemies come storming in. This a good to know, especially when it comes to Boomshields. Boomshields are probably the most useful weapon in Horde, since they will block off a number of enemies. When a new round starts, make sure there aren't any loose or planted Boomshields around as they will disappear. After you start hearing enemies, plant the shields where you need them.
Some enemies can kick down the shields, so to prevent it, simply plant the shields backwards (so, the outside part/black and red part) is facing towards the area you're blocking off. Boomshields will normally fend off any of the Boomers, Sire, Kantus, and Bloodmounts. The drone varients (Grenadiers and Theron Guards too) can jump over them if I remember correctly, so watch out. Make sure you have the shields planted well enough, sicne the last thing you'll need is a storm of Maulers and Bloodmounts killing off your entire team in one swoop.

Planting grenades in the walls isn't a bad idea, but I found that Tickers and Wretches will set them off intentionally. When it comes to the enemies themselves, some will take priority over the others. Kantus will have to be killed before focusing on any of the drones (unless you're popping headshots. Then the Kantus can't do shit about that). Flamers are easy to kill by just shooting their Imulsion tanks. Grinders and regular Boomers should probably be disposed of first. If you have the shields set up, Maulers, Butchers, Sire, and Bloodmounts will just chill next to the shields. They are pretty easy to kill in this state. Just don't get too close to them. Wretches and Tickers should always be taken care of last, and they will also hunch up by the boomshield locations.

Now, I can give some tips for different levels.

Avalanche: I dislike this level because of the lack of decent weapons available from the get go, but it does have a nice place to make base. The base for here should be the upper area where the Mulcher/Mortar spawns. If you're playing with five people, have two people cover each stairwell (with boomshields, when you can find them) and have the fifth person use the mortar and mulcher. They should help out both sides.

Stasis: This level is kind of tricky, but when I played it, my group took over the bridge where Boomshot/Mulcher spawn. We used the same strategy as we did for Avalanche, but there isn't much cover on the bridge. Also, enemies will get stuck in spawn, so you will have to break formation to kill off everyone else in certain rounds.

Security: This is a popular level because of the lasers, and it makes sense. Enemies will stay away when you have those lasers up, but they will turn off the lasers every now and then. Ideally, I would have three guys inside of the laser room and two guys making a base by the outside switch. So, three guys would cover the area where the Mulcher/Mortar spawns. To prevent the Locust from messing with the lasers, two guys would put up Boomshields at the entrances to the outside laser switch.
However, we have only focused on just having everyone inside of the "laser room" whenever I played here. We would round up ammo when there were only a few enemies left and place grenades outside of the laser doors to prevent enemies from coming in. The boomshields will work, though. If you happen to get stuck outside of the laser room and your team doesn't want to let you in, you can always smoke grenade youself into the room...but if you mess up, you will die (stand by the lasers, toss a smoke at your feet and hope you bypass the lasers when you fall over).

River: It's all about the barns here. Pick a barn and block off the entrance with two boomshields (three if you feel it necessary). The barn will protect you, but it does have its problems. The upstairs area is 100% safe. Grabbing the Torque/Longshot could prove fatal. Also, the bottom floor has very little area for cover. You could try using the Mortar from upstairs, but it could be too risky. Enemy gunfire and the little bit of roof the juts out make the risk greater than the reward.
I suggest putting a few frags outside of the barn. Something to watch out for, though, is Boomers. They will be very keen on blasting you with their Boomshots and considering the barns offers very little in terms of cover, things could get chaotic. Be sure the Boomer doesn't happen to shoot any grenades you have planted inside of the barn either.

Day One: I found this to be a fun map to play on. I'd suggest splitting up into two teams. One team should go to Torque/Longshot spawn and the other team should go to Hammer of Dawn/Mortar spawn. Someone should set a shield on the stairs for both sides. Then its all about killing everyone and enjoying the high ground.
Grinders and Boomers will be a problem, especially if they get in close. A tip for the Hammer of Dawn: Since it has ammo in this game, be sure to just hit enemies with the intial blast. Don't hold the beam until it stops, since the beam gets progressively weaker the longer it is held. Just get the beam to hit, wait a half second, and let go. This'll save on ammo and make it more damaging. Also, be sure someone isn't interupting your beam by standing in front of you and aim at the ground of the enemy, not at the enemy.
Mortar users will have to be careful of where they are aiming. You don't want to waste shots by hitting the various buildings.

Gridlock: This is probably my favorite level for Horde. I'd like it more if there was a Hammer of Dawn somewhere on the map.
The idea here is to make your base at one of the two pistol spawns. Your team will be split into two: Three people will cover the "outside" area by the stairs and two people will cover the "inside" area closer to the spawn point. You'll be able to hit a majority of the enemies, and a number of enemies will get caught between the stairs with the boomshield and the wall behind it.
Ammo tends to be easy to get in this level, since there is an ammo box right by the stairs and a boomshot in the middle. Sniper/Torque can be a little risky, but they do help if you can get them.

But yeah, I probably just killed off a bunch of readers by off of this text, but I hope something I said may have helped someone. Have fun!

EDIT: Okay, so my guide is a wee bit outdated. Here are some new things.

-This achievement is obtainable on ANY map. It's possible to get this achievement while playing on any Combustable, Snowblind, or Dark Corners map.
-The Boomshield trick I mentioned has long been removed. Regardless of which way anyone plants the shield, the Drone varients will kick them over. This can be particularly dangerous if a Cyclops does it since they will immediately attempt to chainsaw someone. However, the Boomshield is still the most useful weapon in Horde mode. Tickers, Wretches, Kantus, and all the of Boomer varients will be unable to knock them down.
-As always communication is key. I would recommend playing with people who enjoy Horde mode.

Pick a map you are most familiar with. Find a place in your map with only one or two exits and a spawn point for a large weapon. Use this as your base.

I did it on Avalanche with 1 to 2 other people. There is a raised area along one side of the map where the grinder and mortar spawn.

Make sure you grab the boomshields when they drop. These are essential in the x8, x9, x0 levels. Set them up at the entrances to your base so the raiders (beast riders) can't get in. Then shoot over them. When you've set up shields don't shoot the guys with shields next to them or it'll screw up your existing shield. Just leave them for last, they wont go anywhere.

Another use for the shields is to block a raider (or two) so your friend can shoot them from behind you.

Remember tho' that the shields will disappear at the end of the wave *unless you pick them up*. So be sure to pick them up and then re-set them.

I find Underhill is a good map to do this achievement on. Of course it requires you to have the Snowblind map pack, but if you don't have it then another solution will be for you.

You can either camp in the Scortcher room and put a shield or two at the enterance (make sure the shutter is down), or at the top of the hill where there is a lovely Mortar. You can use the Mortar to take out large enemies as the make their way up the hill.

By doing it on this map you'll also get the achievement for doing all 50 waves on a Snowblind map. So you are killing two birds with one stone.

Me and a friend, just the two of us, managed to do this very easily by camping in the room. There's an ammo box just outside too to top up at the end of each round. Shields at the doorway stop even Boomer shots coming in, all you have to do is stand back and shoot, keep picking up that Scortcher.

Good luck.

Me and my Friends got this achievment on the map called SECURITY, get the frags/inks and head into the area where the mulcher turret spawns, when the lasers are up the horde wont approach, use this time wisely to take out the big guys, every now and again a locust will the turn the lasers off, just defend your ground and wait for the lasers to turn on. it would also help if you can plant BOOMSHIELDS where the lasers are so when the defences are down, big guys wont get close to you, GOOD LUCK

I Find River to be a Great map to get this Achievement on as the two barns on either side, offer great cover and distance from the enemy spawn points, this is good as the Longshot and Torque Bow spawn on the upper floor of either barn, so you can take out a fair amount of Horde before they reach you. River is also good because the enemies thankfully have to come to you, you dont have to go to them. As there is only i entrance that the Horde can come through, it is essential that when able to pick up a Boomshield or if with a friend both pick up shields as you can use these to block the one entrance. If you repeat this wave 50 should be a walk in the park to achieve.

The best map to do this on is river. Camp in one of the buildings where sniper/torque spawns. One person covers the door whiler the other snipes. If you have 5 use one building for each pair with number 5 free roam. Use boom shields to block stairs and slow enemies down. If you struggle with the later waves try day one and go to the sniper torque bow spawn there. There is a little barricade so none of the maulers (big guys with shields) will be able to reach you and you can be safe withouth needing boomshields. A way you can get two in one as mentioned in another solution is by doing avalanche. There is a place with a garage door which opens and closes camp in there. There is a boomshield nearby which you can use to guard the entrance. After every wave there door either opens or closes depending on what state it was before the wave ended so make sure you open it with only a few enemies left so it closes at the end of the wave.

Me and my friends were able to survive all 50 waves in one sitting by playing on Pavilion. We all entered the room with the shield and planted the shield (hold LT and press A) in the entrance to this secluded area, since there is only one entrance. Then we just camped in that room. The larger enemies like Maulers, Boomers, Butchers, Flamers and Grinders cannot fit through the small gap leading into the area. Also the shield blocks all the Grinders bullets and the Boomers rockets. Every so often make sure you pick up and replant the shirld to make sure it doesn't disappear. I would suggest getting a Grinder's mini-gun every so often to save some ammo too. Finally, there is an ammo crate right outside the room so if you get low you don't have to run miles to get one.

Good luck and have fun!

Go to day 1 hide in the corner next to the sniper and shoot for hours on end. watch for guys jumping the wall and replenish torque bow n sniper rifle ammo at the end of each round. hold on to that shotgun people.

This is how me and a few guys go the achievement a couple of days ago. This was done on casual: There were four of us in the Pavilion in a small room in the middle of the hallway that holds a boomshield. We would plant the shield and fight, but it would get kicked down.
What we did instead was use the Living Shield method, in this method you have one person who stays in the archway between that small room and the hallway to block incommin attackers while your teammates kill everyone else. The shileder is immune to almost everything, except for ink grenades and chainsaws.

I done this with my brother on Blood Drive Map.

Go to one of the two staircases,climb up until the end then turn into the dead-end alley (right or left,depending which staircase do you choose) them plant a boomshield on the entrance and hide on the last column,right near the pistol respawn (it can be a gorgon pistol or a boltok pistol,depending in which wave you are).

Now just get concentrated and comunicate with your partners.

If you still didn't get it,here's a Youtube strategy guide video:
( ALL CREDITS GO TO GAMERBOY4000)