Fleetfoot Achievement in Octopath Traveler

For this achievement, you must beat the game in under eight hours. This sounds difficult, but you only need to complete the four chapters of one specific character's journey to accomplish this. In your main playthrough, you'd be recruiting all characters and doing a ton of extra content that can be skipped here. You will want to couple this achievement with a solo run to unlock Lone Traveler (50G) at the same time. While this does make the speed run slightly more difficult, it is still entirely doable. In fact, you can complete it in about 5-6 hours fairly easily with Tressa, and each character generally has a viable option to do so as well.
 

The guide found HERE will lay out how to do the solo speed run. It focuses mostly on Tressa, but does offer tips on other characters as well.