You can get this by the end of the story.
I'd done all liberations upto the last one on the final Island, all of Masako's tales, all of the Mythic tales, also any side tale that was available to me, and it unlocked for me when I hit the mission for battle preparations to attack
Here'a a list of things that are useful to know.
• All main quests give a lot of progress
• All Mythic, side and ally quests give off a moderate amount of progress
• Killing Mongol/Straw Hat patrols gives off a small amount of progress
• Saving civilians from Mongol's gives a small amount of progress
• Liberating Mongol Territory gives a moderare amount of Progress
• Mongol Territory bonus objectives gives a boost to the above.