The final boss fights against the Order of the Circle. There are five bosses for the finale of this game.
Set up for this fight:
Level 68:
Team Lann: Ifrit, Lann, and Serafie (Holy, Flare, Fira, Haste, and All Resistances Mirajewels)
Team Reynn: Tamamohime, Reynn, and Black Chocochick (Curaga, Libra, Faith, Agility+ and Flare Mirajewels)
Boss 1:
The first boss of this is Segwarides with just Lann. Much like the first fight he is weak to light-elemental attacks, but at this point using normal attacks on him will be the best way to go to avoid needless wasting of AP. The main thing to watch out for is Disaster, which inflicts a ton of status ailments on you, so have Remedies ready. Heal during the fight and keep attacking him until he dies.
Boss 2:
The second boss of this is Pellinore with just Reynn. Like the previous fight against her, she is weak to water-elemental attacks, so Water + Faith is a good option for this fight to inflict about 5000 damage in each hit. She should eventually go down fast with this strategy.
Boss 3:
The third boss is Pellinore against Lann and Reynn together. She has a new aoe attack called Tri-Disaster that deals damage to the entire party, so minor healing should be needed there. Other than that, same for the previous fight applies here, use Water + Faith and use your water-elemental attack items like Water Spellstones to deal heavy damage to her. Use hard hitting attacks also like Tama's Blossom Storm with Faith to deal more damage. Eventually she should go down.
Boss 4:
The penultimate boss is another fight against Brandelis. The encounter is exactly the same as the previous fights with him. The main thing to watch out for this fight is his Arcarmament 1-4-6, which does heavy damage to the entire party, so be sure to have Curaga ready to heal through the attack. His Double Cut attack deals four small physical attacks to a single party member also. He has no elemental weakness, so hit him with everything you have until he dies. This is the last fight that allows you to use your champions, so make use of them and lay down heavy hitters with the summons. When he reaches critical HP he will have Haste automatically casted on himself, so have Dispel or a Holy Torch ready to remove it.
Boss 5:
Lastly is the final boss, Exnine Bahamut. This is indeed a tough fight if approached the wrong way. He has access to powerful Ja-tier magics, status inflicting skills like Dark Revels, powerful aoe moves like Chaotic Impulse, and moves that can knock your stacks apart. Be prepared to use status ailment recovery items and healing moves such as Curaga and even your aoe healing items.Worst of all your champions are sealed for this fight, so no free hard hits from them.
Best thing to do here is open up by using Haste, Protect, and Shell stones on the entire party to avoid wasting AP on the spells. Cast Faith and Bravery here to increase your damage output. Using physical attacks when charging your AP can be a good starter, but once your AP is full use skills like Tama's Blossom Storm+ for a good 8000+ damage to his health.
Like the fight before, he will gain auto-haste on himself to speed up his turns. So have Haste Stones ready to counter that. At a certain point he will begin to do a Megaflare-esque countdown to his special attack Megaflare Cannon, which deals heavy damage to the party, so use the countdown turns to prepare to mitigate damage. Once he's done using that, heal up.
Highly suggest for the last 15% of this fight is to use the Bahamut Mega Mirage. To lay down an easy 10-12k damage on the boss to finish him.