You can unlock this during the "Weight of the World" portion of the story if you're extremely thorough! Otherwise you'll need to invest extra time after the game for those extra Guild Seals.
To pretty much complete the whole of the Road to Rule you'll need to complete most, if not all optional quest. Get additional Seals by interacting with people, befriending them and performing expressions!
Do not shy away from combat as you can earn your fair share of Seals from fighting alone!
For a full layout of the Road to Rule please see the Road to Rule Guide.