Like a Dragon: Infinite Wealth

This achievement is rewarded after completing the first sector of the Big Swell dungeon and watching the scenes following that.

The Big Swell itself is a dungeon that is only accessible during Premium Adventure in Honolulu. The first sector itself has a recommended minimum level of 55. The main thing to keep in mind is the boss fight, which is against...

, to be more precise, three thought entities of him (much like most bosses in the Final Millennium Tower were). Each entity has their own elemental weakness (blue: electric, yellow: fire, red: ice), and are neutral on physical weaknesses. They also resist their own coloured element.

As for the fight itself, keep your party healed up as best you can and focus one entity at a time. Saeko as Idol is fantastic for healing and clearing off any status effects you want gone. My other party members for this were Ichiban and Kiryu both in their default class (with Burning Arrow inherited from Samurai), and Seonhee in her default class (with Winged Kunai and Illusory Slash inherited to abuse her increased effectiveness on agility buffs).