Gears 5

To get this achievement, you must have 5 players to achieve this. You want to select horde, go in a custom match, host game. After that set the difficulty to “beginner.” Once your difficulty is set, click on “horde,” that’ll put you in the lobby’s settings. From there under “horde options” set starting wave to 10, this will put you on a boss wave automatically. The preferred setup of characters is characters with duration ultimates. Marcus, Kait, Fahz, Del, Emile, Sarah Connor. Kat has duration, too. Kat is not on the list because her ultimate doesn’t last long but is doable. Once you got your compilation of characters, pick any map and put the fabricator inside of a building. Once the wave starts proceed to kill all minions and start damaging the boss until it gets it’s health down to about 5%. Now, have someone count down from 3. After the timer, everyone activates their ultimate and immediately kills the boss. If one player is playing kait I suggest that player lure the boss around. (if it’s a Matriarch) Another note: If a player is playing Kait they CANNOT shoot or this deactivates her ultimate. You need to kill the boss WHILE her cloak is active. Simply put, as Kait, don’t shoot and let your other teammates kill it.

This is pretty easy, but with this setup it's even easier I believe.

You need a full team of human players to get this achievement. You can have players playing with 2 controllers (split screen).

Set up a private horde match:

Map: District
Start: Wave 10
Difficulty: Beginner

For characters, I would pick Kait, Marcus, Del, Fahz and JD.

When the map loads, pick up the fabricator and place it inside the building. When the wave starts, you can just go for it with any boss you get or you can die until you get the Kestrel (easiest boss for doing this). It's also been tested with the Swarmak and Snatcher, and it works with both of them (thanks Ham Man 89 for the info)

Once you get the Kestrel, kill all enemies BUT the Kestrel and then gradually lower its health until it's really low. As Ham Man 89 mentions in the comments, don't use bleed damage cards and make sure everyone shoots the boss at least once before activating your ultimates.

At that point, JD spots it, making sure he has a clear view of the Kestrel (it should still be spotted by JD, no one else should spot it) and someone starts a countdown for everyone to press and activate their abilities . JD's ability alone will take care of the boss, if the health bar is low enough, so there's no risk of any ability running out before you take out the boss. It might also be a good idea for JD to wait a couple of seconds to activate his ultimate, to account for any connection lag if there's people from different locations in the team.

Some notes:

When I did it, one of our guys had picked Kat (from Halo), and everyone but him unlocked the achievement. This was probably due to synchronization issues rather than because of the character he picked (no one should have unlocked the achievement if it's the character that voided it for him), but just to be on the safe side, I would go with a main horde character (with the exception of jack probably, given how his ability works).

Also, make sure you know how to use your ability (for instance, you have to spot someone for JD's to work, you cannot shoot while your ability is active if you're using Kait, etc)