This is obtained after the following through the main storyline and completing the final quest. Its is awarded after the ending scene.
Make sure your well stocked with ammo and stimpacks going in.
The finale takes place in Ulysses' Temple. You go through a minute of dialogue and then must decide how to finish him off.
*The optional quest is to disable his eyebots which make finishing him significantly easier. The terminals for each are found on the second level on each side of the rocket. It requires a SCIENCE or REPAIR of 100.
After dealing with him and a dozen Marked men, you will head over to the console and decide how you want to end the game.
*As indicated by BillJones0302
If you have a SPEECH skill of 95 or higher you can convince him that he is "on the wrong path". He will then join your side and help you defeat the Marked Men. After the fighting he will go his separate way.
*As Indicated by Kinesis916 & daevv
You have to cancel the missile launch, otherwise you fail the mission.
Launch the missile at the Legion and after the ending you will obtain the achievement plus a new area on the main game will open up.
Dry Wells: http://fallout.wikia.com/wiki/Dry_Wells