This achievement is acquired later in the game but MUST BE someone other than the main character.
The max level is 24/25, so plan accordingly so your skills aren't too stretched out. This achievement should be marked missable, honestly.
What I did is simply stopped unlocking talents for the characters I didn't use once I had every party member available. By being proactive to get every possible secret and defeating all mobs up to the last area of the game I was level 23/24 and easily popped it with an inactive character.