● On the main menu select "Multiplayer" and then select "Custom Match" with no opponents.
This way you can spawn units without any threats.
● Select the "Feeding Area" map and choose whatever faction you want to play as.
● This map has 5 fan geysers, make sure to take over them. As soon as that is done, use the "Summon Flag" solo, and put it in a range where the Lamprey (gigantic tapeworm-like creature) can kill your units so that you can make space for more units to spawn.
This achievement will take the most time to complete (~6-8 hours) so feel free to entertain yourself with something in the background (e.g. watching a YouTube video or a series) Don't forget to check back on the game every 10 or so minutes.