This achievement is obtained by reaching one of the four faction endings
for the main questline of the game for the first time.
I made a chart that shows how to obtain all the achievements with only one save / reload
, and also the best point to save in order to obtain all 4 endings (not necessary for achievements).
Scroll down for a more detailed explanation (inside the Spoiler Tag, as it is an awful lot of text).
For starters, keep in mind a thing: having high charisma
will help you when you'll have to balance several factions at once. I recommend having at least 6-7 points of charisma, and using "charisma-enhancer" gear like Glasses, Hats or Suits, or even using, when neccessary, beer or some drugs as Grape Mentats or Day Tripper that give charisma points for a few minutes.
Open this thing for the full explanation:
Hope it helps!