The Strongest Sojourner Achievement in Octopath Traveler

To deal maximum damage, you must first ensure the character you are using has the Surpassing Power skill equipped from the Warrior class. This allows you to go over 9999 damage. Next, be a very high level. If you do this after defeating Galdera, it will be extremely simple. Go back to a Level 11 danger area, outside one of the starting towns. Max out your BP with items or defending a few terms, then use a combination of strength-boosting buffs on your warrior and defense-weakning debuffs on the enemy and unleash a max level attack. This will easily do the full cap of 99999 damage and unlock the achievement.