At the time this game came out, the path to getting these cheevos was much more nebulous. Now that I got everything on Last Light, it was easy to go back and get enlightened, which I just finished: listen to conversations, refuse ammo from civilians, follow instructions from Khan, and run towards the Dark Ones in the visions you get.
You can loot anything and everything, dead civilians and guards a like. That makes no difference.
Whereas Last Light really made it obvious that you had to explore to get good endings (finding secret rooms and the like), this one mostly focused on decisions you make in Metro stations and following Khan's directions.