The Luminous Magic Beast is one of the final bosses in Lost Odyssey. It is summoned by Gongora when the heroes confront him in the Tower of Mirrors, and is said to be spawned from the magic energy emitted by the mirrors within the tower.
The beast will open the battle with Ray Charge, which allows it to absorb magic attacks. Because of this, consider using Tolten or Sed as a physical attacker - You'll definitely want Sarah and either Ming or Jansen on the team. Have them cast Powerus on both Kaim and Seth (who should be wearing rings that give them the Beast Killer skill) to maximize your damage. Next, have Jansen and Sarah get up All-Barricadus, as well as get Covera on the physical attackers. Once that's done, they can just sit back and spam Zephyra - you may not need it when they start casting, but it gives some insurance against the boss's bigger attacks. If you have it, All-Generate can work nicely as well.
Note that having the Magic Counter ability equipped on any party member is detrimental, since even counterattacks will be absorbed by the beast.
Offensively, the Luminous Magic Beast mainly uses Forceus and the top-tier multi-target elemental spells (All-Flarus, All-Aquaus, All-Groundus, and All-Windus). Any gear that reduces elemental damage is helpful, especially if you've gotten the Absorb All skill for your immortals. Every few turns, the beast will cast Ultimate Ray, which deals heavy damage to the entire party, but proper defensive buffing and healing should allow everyone to survive. After using Ultimate Ray, the Luminous Magic Beast will be temporarily vulnerable to magic attacks (signalled by its tentacles turning red), so feel free to throw a few spells its way if your HP isn't looking too poor.
It might take a while, but the Luminous Magic Beast can be worn down with the above strategies and basic combat tactics. Once it falls, you can finally get a shot at Gongora!