Crevasse : Go along the path, pretty much. Open the chest on the way for a Mega - Potion and continue on.
Road : Go along the linear path again. Speak with the first person along it for 400 Gil , then continue to the Temple. After a scene, you’ll enter.
Hall : Speak with Tromell for a Shell Targe and open the chests in the area for two X-Potions and 5,000 Gil . Approach the steps to the Cloister and someone will come out of the back rooms in horror; go into the room behind them for the reason as to why they’re in horror. After the scene, open the chests in this room for two Remedies and speak with the person who came out of the room for two Hi-Potions . Go into the other set of backrooms for an Ether and an Elixir from the men in there, with three Phoenix Downs also in a chest. That’s about it; enter the Cloister! (Save before doing so, because…)
BOSS - Seymour
This boss can be divided into three individual phases, in the first of which you’ll fight Seymour and two Guado Guardians . The Guados will focus on healing Seymour for 1,000 HP anytime he suffers any kind of damage and Remedy him if ailed. Otherwise, they can Protect themselves, use Shremedy to cause confusion, and use the standard Black Magic (Fire, Blizzard, Thunder, Water) on you. Seymour, meanwhile, will use Blizzara, Thundara, Watera, and Fira, in that order, each for around 500~700 damage. He’ll open with a self-Shell.
For the first phase, you’ll probably want to use the Talk Trigger Command - for Tidus, this is a Strength +10 boost, and a Magic Defense +10 boost on Yuna. Open with Hastega from Tidus as well until you get the Guados’ Auto-Potion stopped: to stop it, simply have Rikku Steal from them! XD Once both enemies are stolen from, you can focus on brute-force elimination. Don’t use Overdrives for now; mostly stick to focused, Magic-based (i.e. Lulu) offenses on them.
And, while all this is going on, Yuna still has to do something: nullify elements! Seymour has a big flaw in his strategy in that he’ll use his spells in a consistent order: Blizzara, Thundara, Watera, and Fira; thusly, have Yuna set up NulFrost, NulShock, NulTide, and NulBlaze, in that order. This fight won’t last long while the "focused offense" group wails on Seymour (who, by the way, should be fought physically due to his Shell), since, once 3,000 HP is gone…
Seymour summons Anima at 3,000 HP. Anima is a very powerful Summon, though this battle fails to show it. Anima will use her (yes, her) relatively typical skill: Pain . This is guaranteed Death, or around 1,000~2,000 damage if Deathproof’d, which is very impossible right now. You can also get hit by her Overdrive, Oblivion , 16 random-targeting hits, which basically means 2,000+ damage to your party.
So, how to go about it? Trust me, Pain is not something you’ll want to stand up against at this point in the game, assuming you somehow even have Deathproof on your armor. Have Yuna open up the fight with her new "???" Aeon, which is actually Shiva . Have Shiva stick to mostly Attack for now. (Heavenly Strike delays you more than Anima since Anima is immune to Delay effects. And Blizzara wastes MP.) If you need healing, cast Blizzara on Shiva, but mostly tough it out with Attack. Once Shiva’s Overdrive is reached, wait for Anima to use Boost: since this boosts her damage intake by 50% or so, you can throw out the Overdrive then for a good 7,000 damage or so. (Or you can save it for later. Whatever.) Anima’s not that difficult with self-curation.
After that, Seymour comes back fully healed and raring to avenge his Aeon! Or something. In any case, nothing’s changed from the first phase of the battle other than Seymour will use his spells in a Multi-Spell form now, able to use them twice now. This basically means Yuna really needs to keep up the work on the Nul-spells, but otherwise little to say.
Speaking of which, I will note that Seymour is suspectible to several ailments you may find useful - moreso in this part than the first (hence the note) due to his redoubled offense, Magic +28% boost, and how you now must eliminate his 6,000 HP in whole. Magic Break may hit (about 50% chance) and halve his Magic down to about 16 (64% of the starting Magic). If you Poison him (close to 50% chance), all you have to worry about is living out ten turns before the Poison kills him. (It takes off 10%, or 600, of his HP per turn.) Just some thoughts.
After the scene, go along to the Save Sphere and save, then head into the Cloister of Trials.