After hours of challenging puzzles in the Temple of Enlightenment , you will face Fu to prove that you are worthy of his Spirit Magic.
The battle will start off with Fu summoning an Obsidian Miasma with 16,800HP. Once it is defeated, Fu will quickly follow up by summoning two Reverse Souls with 22,400HP each. When either a soul or Fu gets low on health, the Reverse Soul will use Sacrifice Self which will heal Fu and summon a new Reverse Soul with full health. Once the one Reverse Soul is defeated, the second Reverse Soul's casting time for Sacrifice Self will be dramatically shortened; counter this by casting Death on the second one. It takes the Reverse Soul two turns to cast Sacrifice Self so you must be able to eliminate it within that time. Once both Souls are defeated, you can focus on Fu himself. He can not heal himself in any way at this point, so you just need to focus on whittling away at his health.