This achievement is story related. Unless you are subjected to a glitch, you will unlock it upon completing the Twisted Religion main story mission during your first playthrough. During the mission, you will come across two access points. Make sure you save. After the boss battle, open your map and make your way to the access point of your choice and make a save. When you exit the factory, the achievement should unlock just before, during, or just after the cut scene.
If you receive the Weird Machines main story mission, and make your way to the Resistance Camp without the achievement unlocking, a glitch has occurred. If this happens, clear the cache. To do so, turn off the console by holding the
on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load the save you made at the City Ruins: Deep Cave before proceeding to the Copied City. Clearing the cache will most likely remedy the problem and you should earn the achievement when you complete the 9S Captured Mission.
Spoilers:
*** Spoiler - click to reveal ***After you finish the Copied City, the Olive Branch Main Story quest has you meet with Pascal at the factory. Enter with him. After following the objective marker and interacting at the quest marker, you will receive a new main story mission--Twisted Religion. The path is quite linear. Make sure you make a save at the Factory: Hangar access point where you defeat a Goliath Biped. The conveyor belt section with the big descending mechanisms can kill you regardless of difficulty. Time your way through them. If you are killed, reload the save file you made at the Factory: Hangar access point or continue the game without reloading and recover your body when you get back to the conveyor belt.
After you clear the room you enter in following the conveyor belt, if your are collecting weapons, do not immediately pursue the objective marker. Instead, turn left after exiting the chamber. You can now, carefully, jump on top of the crushing mechanisms you avoided earlier. Go all the way across them until you get to a room. Inside the room is a chest with an Engine Blade short sword. If you are collecting weapons, this is the best playthrough to get this one. Although it is easier getting most of the weapons in your B playthrough, 9S does not go into this section of the factory in the B playthrough. Accordingly, you would have to wait quite some time until the weapon becomes available again.
Whether you opted to go after the Engine Blade or not, continue towards the objective marker. You will come to the second access point inside the factory--Factory: Furnace. Make a save then take the nearby lift. Follow the objective marker. After the boss encounter with a level 30 Multi-legged Goliath Level 30, open the map. Return, first, to the Factory: Hangar access point and make a save. After saving, follow the objective marker and exit the factory. Going outside, triggers a cut scene and unlocks the achievement.