5/29/12 EDIT
Alright I'm completely scrapping my original solution and linking you to this page. The achievement is based off a points system. The only way to obtain the Enlightened achievement is to obtain a certain amount of points. I just ran through this game this past weekend and I only missed 3 possible points and got the achievement. How many you need is unkown by me at this time
Get the points, Get the Achievement
http://metro2033.wikia.com/wiki/Moral_Points