Defeated a powerful enemy with graceful poise.
This is story-related and can only be done in Episode 2: Bresha Ruins (005AF).
As you progress in this level you will have to weaken Atlas and defeat him. This fight is part of the story and can't be missed.
It's recommended to set the difficulty down to easy in the game settings. In the beginning of the fight you will want to use a Paradigma with Ravagers only. Once Atlas has been staggered you change to a Commando Paradigma in order to finish him as quickly as possible. When his health is down to 0 a little a cinematic action event will start. Press the shown buttons and you're done.
If you didn't get a 5 star rating you have been to slow. You can come back later via time reversal and try it again, so don't worry.