Closure Achievement in Octopath Traveler

After completing all of the Chapter 4 stories with each character, you can take on the true final boss of the game. There are a few other requirements (namely a few side-quests which we'll be doing for another achievement anyway), so finish those up and then get ready for the final challenge of the game. Unfortunately, this requires that ALL characters be around Level 70+ which is frustrating since the majority of the game can be done with a primary group of four, simply subbing in for the required bench characters when doing their chapters. However, leveling is fairly simple while abusing the Dancer job. You can get bonuses of 2x/5x XP/JP fairly often, and if you're extremely lucky possibly even a 100x bonus. You can equip Captain's Badge and Badge of Friendship on your weaker characters to get them more XP, and have the Warbringer's passive skill to increas XP for the entire party equipped as well. See the video below for this method.

A more brute force method to leveling is simply repeating battles in the Forest of Purgation. Use your sorcerer to one-shot all these enemies for nearly 1-1.5k XP per battle, with chances to get Cait enemies for around 3.5k XP per battle. This should be almost as fast as the above method, and stays reliable the entire time which is nice.

Once you've gone through and leveled all your characters up, you can tackle the final dungeon. The walkthrough HERE has a very good rundown of this boss' abilities and phases, but after many failed attempts using their strategy, I searched around for additional help and found that it is missing a key step to make the souls in Phase 1 irrelevant. You will want to capture at least two copies of the Devourer of Dreams enemy using the Hunter class in the Everhold Tunnels. You must first defeat this enemy as the mini-boss of the dungeon and then it has a low chance to show up in random encounters. The great thing about this capture is that at MAX boost in battle, it will one-hit-kill any enemy aside from a boss. This means it will wipe out all of the souls every time they are renewed, allowing you to just continually break and damage the boss! What an absolute relief it was when I did the fight this way.

The second phase can simply be demolished with Oberic after that. You'll generally want him at 1HP and hope he doesn't act first in battle which will allow someone else to feed him a BP item, allowing him to use the Warmaster special attack. This should kill two of the three body pieces (usually the Blade survives), and your other characters can finish that off before the next turn. Continue to feed Olberic BP and after 2-3 more rounds the boss should be dead. For reference, my Olberic was Level 82 while every other character was low 70's.