|You'll first have an open-world fight against two Aether-starved Soulflayers. Be careful of the Goosefish nearby that can easily aggro midfight, and feel free to pull out your chocobo. Once the fight is over, use your remedy on Sanche and the real fight will begin.
You'll face off against Camazotz. Sylphie Sweetwind and Brother E-Sumi-Yan will tether Gatty Deepwood to help her with her trance. Shortly into the fight though, Camazotz will use Corrosive Gale and knock out Sylphie, and Gatty will go out of control and use Aetheric Burst, greatly hurting you, your allies, and herself. Simultaneously three Planar Fissures will appear and begin using Void Call, and Camazotz will become untargetable and tether Gatty, inflicting her with Life Drain.
You should take out the fissures ASAP, the smaller ones will summon an Aether-starved Hecteyes while the larger one will summon an Aether-starved Arch Demon. The smaller ones go down more easily but the larger one summons the bigger threat, so choose which to go after (It should be noted that the guide writer did not focus the larger one, but with the mechanics it has it may be impossible to deal with and one will always need to fight the arch demon). The Arch Demon will eventually use Abyssal Transfixion, dealing damage and inflicting Paralysis, but if you set Esuna you can remove it. It will also eventually use Abyssal Charge, summoning four Abyssal Lances that will charge at you, and a pair of Planar Fissures will begin using Void Call. The fissures will summon more hecteyes and should be your first priority.
Once they are all taken down E-Sumi-Yan will LB, using Pulse of Life to revive Sylphie and fully heal you and Gatty. Sylphie will be revived at 1 HP however and be Down for the Count, so toss her a few quick heals. Once she's at full health she will go back to tethering Gatty, who will soon regain her composure and knock down Camazotz with a flash of light. Once the boss is targetable again just wail on it and it should go down fairly quickly.