Story-related and can't be missed. To get it...
*** Spoiler - click to reveal ***You must retrieve your first "Oz Kit" for Springs in Serenity's Waste during the final leg of the "Lost Legion Invasion" Mission.
It can be found in the slightly-Northeast section of the map behind a door with 4 locks (next to an Air Dome Generator). Smash the locks with your melee attack to get inside
Once you turn the mission in, the Achievement will pop.