Instructions
Accept the "God or Devil?" petition from Montblanc at the Clan Hall in Rabanastre. He will direct you to the Pharos at Ridorana.
Head to the Penumbra floor at the Pharos. Begin collecting black orbs by defeating enemies as you make your way through the level. Place the black orbs in each of the four altars in the corner rooms of the floor. Put 18 orbs in the northeast altar, nine in the northwest, three in the southeast and six in the southwest.
Search the rooms on the north side of Penumbra until you encounter the Phoenix. Defeat it, then take the elevator to the Umbra floor.
Continue acquiring black orbs from enemies on the Umbra floor and put them in the altars here. Place 15 orbs in the northeast altar, nine in the northwest, 15 in the southeast and 18 in the southwest. Take the elevator to the Abyssal floor.
Make your way through the Abyssal floor, defeating enemies and collecting black orbs. Place 27 orbs in the northeast altar, 12 in the northwest, 21 in the southeast and 15 in the southwest. Head to the elevator and prepare for battle. Cast Protect, Shell, Haste, Bravery and Bubble on your characters. Ride the elevator to the Unknown floor, where you will encounter the Seer.
Begin the battle with the Seer by casting Dispel. Move in and begin your attacks. The Seer will become immune to all attacks and begin summoning his minions for support when you've depleted 20 percent of his health. You will have to fight Pandemonium, Sylt, Fenrir and Phoenix in succession. The Seer will cast Dispel on your characters to remove their statuses during the battles. Recast as needed and continue the fight. You will be able to attack the Seer again when you've defeated all four of his minions.
Refocus your attacks on the Seer. His spells will be stronger now, so keep your characters' defensive statuses active. Monitor your health as you continue your assault and the Seer will eventually fall. Return to Montblanc to collect your reward.