UPDATE: FINALLY FIXED WITH LATEST UPDATE! (4/27/2017)
SUPPOSEDLY FIXED! (5/17/17) There appear to be still issues with getting this to unlock! Please comment with how you got it to unlock if it didn't unlock first try for you!
------------------------------------------
This is far easier than it might sound, because you can put the AI on EASY!!
Find a buddy* to do this with(make sure they're on your friends list just in case that matters).
*Thanks to Dritnul for confirming you CANNOT have an AI partner along with you and your buddy to get this achievement!
Load up a custom skirmish game, put the AI on easy, game mode Deathmatch, Fog of War off, max resources.
When the game starts build on every building point on the map, then build up and rush the AI however you want.
Flamethrowers(Barracks) are a good anti-building rush unit for the UNSC and Brutes are a good anti-building rush unit for the Banished.