To unlock this achievement, for 10 characters you have to have reached the end of the timeline bar on the Soul Chronicle selection screen, and all of the icons have the be dark blue color. Some characters have "stone" icons that lock the parts from being playable. To unlock them, you have to do story parts for certain other characters. As Groh and Azwell are related to many characters, doing their stories first unlocks a lot of others.
Zasalamel has no fights and is all text, so if you want 10 faster, you can do his as one.
Tira and any additional DLC characters do NOT count for this achievement.