Story related and cannot be missed on default settings. Soon after completing the tutorial or starting your playthrough you will be advised you will be doing a Faction meet-up. Here you will have to take 3 sets of 2 soldiers. The mission will then be in several phases, upon completion of the final phase when you reach the evac zone with all soldiers the achievement will pop shortly after.
If you choose your own faction at the start you will not get this mission pop-up hence the missable tag.