Currently unobtainable, like the other online achievements in the game.
Per the discussion
here, it appears the servers are down as of 28 July 21. I'd suggest monitoring that thread and/or the list of Recent Winners on the right side of this screen to check if people are able to get online.
Servers are back up, as of 07 June 2021. Having been down for a long time, I'm not sure how long they'll be back, so if you want this achievement I suggest getting on to it right away. You will need a boosting partner or a second Xbox and a dummy account (can just be a silver account with game sharing) to unlock this achievement.
Given that I was trying to get the online achievements done asap without having actually played the game, this was the most confusing for me, hence the solution.
From the main menu, select Skirmish > Create a Mission.
You will get a notification that servers are offline but the screen should load just fine.
You will see a set of tiles, each with a mission. You want to uncheck the "Official" box on the menu on the left side. Look at the bottom right of each tile - anything here should do but make sure it
is not by Cyanide. The shorter (less turns) the better but it doesn't really matter.
Pay attention to the information on the right side of the screen. The mission objectives shown there have the win or lose conditions for each side. Find one that can be satisfied by simply reaching the turn limit - eg. Terminators must kill all enemies, Genestealers must have at least one unit survive - in which case the Genestealers will win.
Start the mission. Towards the bottom-centre, change the top row to Private. Assuming
you are getting the achievement, select "Swap Factions" so the team on the left is the one that will win once the time limit expires, usually Genestealers (Terminators are blue and Genestealers are yellow). Now select Create A Mission.
Press
and invite your boosting partner, then have both of you select "Start".
Once you load in, both players will press
until the number of turns (shown to the top-centre) expires, granting an automatic win to the relevant side.
Repeat for your boosting partner and you are done!