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.