This achievement is "Is...Is That All?" and is unlocked by unlocking the True Ending story in the Story Mode.
In order to do this, you must get the True ending for Ragna, Noel, Jin, Rachel, Hakumen, Hazama, and possibly Tsubaki in each of their story modes.
With the exception of Hakumen (who only has the first two), all the characters have 3 endings: True, Bad/Alternate, or Joke. You know you get the True end if at the end you see "Storyname" END for each character. If you get "One of the many possibilities.. of the CONTINUUM SHIFT" and a "Help Me! Professor Kokonoe" section, that means you've gotten the Bad ending. The joke endings are obviously so, usually breaking the fourth wall or having characters acting differently than you'd expect them to.