Master of Solistia Achievement in Octopath Traveler II

This achievement should come naturally as you complete the game and all the side-quests. I can't think of a specific area that wasn't visited along the way to completing the other achievements. With that said, I think the best resource I found was in the Steam Guide HERE, which has screenshots of each region and all the various areas that need to be explored for the achievement. The video below has all of the areas pointed out as well, but some of them have specific requirements to reach which are pointed out in the written Steam guide that you'll want to reference.