This is Mt. Gagazet, home of the Ronso.
Ascending Mt. Gagazet
First off, just run up the slope between the stone columns and you'll have a rather lengthy scene with the Ronso, and particularly how they don't like you being there. However, Yuna will stand her ground and they'll let you pass onwards. Unfortunately, it seems that Biran and Yenke still have a problem with Kimahri. As you try to press further, they'll stop him, and a battle will being.
Sub-boss: Biran and Yenke
Fighting these guys isn't too difficult providing that you remember the one important fact: you only have Kimahri to fight this battle with. Before you complain saying that this makes it absurdly difficult and that this guide never gave you any warning, it's not a problem because they're scaled according to Kimahri's stats. So no matter how overleveled or, most likely, underleveled Kimahri is, Biran and Yenke are no more difficult.
Biran and Yenke together have weaker versions of most of Kimahri's Ronso Rages. The only ones they don't have are Jump, Seed Cannon, Bad Breath (which has little effect on these guys, unfortunately) and Nova (which Kimahri can't have learnt by now anyway). All the others you can learn off these guys if you haven't got them already. However, two new ones (excluding Doom, which you can learn from Ghosts in the Cavern of the Stolen Fayth if you went there first) can only be learnt when the two Ronso are in critical - Mighty Guard, which you perhaps saw Defender X use, can be learnt from Biran, and White Wind can be learnt from Yenke when he's in critical. Both of these only apply to them once they have used their respective attack, then you can learn it off them using Lancet. Otherwise, repeatedly use it until you've got all of the Ronso Rages, making sure to use Kimahri's overdrive up after you get each since it'll be a waste otherwise. Also, Biran may use Berserk instead of Mighty Guard, particularly if you kill Yenke off first, so go for Biran.
Yenke will have about 25% more health than Kimahri does. It's a little less than that in actuality. Biran has about 55-65% more health than Kimahri. Their attacks, as outlined before, are most of the Ronso Rages, but also Bulldoze, which does about 600 damage from Biran and 75 from Yenke, and puts the user on the other side of Kimarhi, rendering most of the Breath Ronso Rages ineffective on one of them. It shouldn't be too difficult to be doing about 700 damage a hit to either Ronso, but make sure you focus on one of them before the other (Yenke is the easiest to kill first). When the two of them are together, they'll guard each other reducing your attacks to about 30 damage to the wrong target. Thus, you want to wait until they're standing on opposite sides of Kimahri for full damage. Next, it is important that you heal Kimahri whenever his health goes below 1300 or so. You will probably have to dig into your X-Potions to do this, but it's necessary to stay alive.
If you get the chance also use Steal - you can get some Level 3 Key Spheres off them which is really helpful for when you get to Zanarkand.
The Mountain Path
Once you've beat them, Biran and Yenke will acknowledge Kimahri's strength and let you all past. In the next screen, hug the right-hand side of the path until you can get on to a higher section of rock. Then head down and around the higher rock to get to a chest containing a rather generous 20000 Gil. Head along and you'll be stopped by the Ronso again, only this time they'll sing the Hymn to you. Once they're finished, hug the left as you carry on, and you'll find a side area with a chest containing 2 Mega-Potions. There are some more enemies on the way up Mt. Gagazet:
- Bandersnatches are wolves, so deal with them using Tidus. Tidus might not be able to kill them in one hit, so be prepared.
- Nidhoggs are armoured dragons. One hit from Auron will dispatch them, but watch out for their physical attacks and whatnot since they hurt.
- Grats are plants similar to the Ragoras you found in Kilika Woods and the Sandragoras in Sanubia Desert. They have two attacks, Seed Cannon (which you can learn off them with Lancet from Kimahri) and Seed Burst, which inflicts damage and confusion.
- Grenades are bomb-type enemies. Hitting them with Auron is the best way to deplete their health in the three attacks necessary to kill/capture it before it uses Self-Destruct (which Kimahri can learn off them).
- Mech Leaders are machina, so use Steal. Watch out for their Electrocute attack.
- Mech Defenders are machina, so use Steal. They're more powerful than the Leaders but slower.
It's a good idea that you equip your Capture weapons and use them as much as possible to save time for you later. Also, if you're underlevelled, and don't make a habit of healing after every battle (either using Yuna or Hi-Potions) then one Ambush can mean the end of you.
Anyway, up ahead you'll find a grave of a summoner and guardian team that failed climbing Mt. Gagazet. Follow the path left and you'll come to a small path around a section of rock, where Braska's Sphere lies for you to watch. This counts as a Jecht Sphere so now you need one more and you'll get Auron's next Overdrive. Follow the path along - it's pretty linear until you get to the next grave. Go left and you'll find a Defending Bracer in the chest at the end. Backtrack and head north this time. Along the next path you'll meet Wantz, O'aka's brother. He's got some awesome equipment but sadly a lot of it is far too expensive for you to buy sensibly, and you'll want to keep around 200,000 Gil in reserve for getting Yojimbo from the Cavern of the Stolen Fayth when you get the Airship, which means you can't afford to shell out for all of the stuff he has. Now may also be an excellent time to sell a lot of the junk you have, just be careful not to sell any of your good stuff. With all the crap you have by now you should be able to get 20,000-30,000 Gil. Anyway, carry on along the path. After heading down you should be below where you were before. Head right along the semi-visible path and around to a secret ledge containing a HP Sphere and a Level 4 Key Sphere. Now, head left and along until you reach the next grave. There's nothing along any other path, so follow the red arrow up to the Save Sphere. Save, but before you press onwards, fight random battles until you've got all your aeons in Overdrive. As you might have guessed, and old...friend...will show up in the next area. It's Seymour!
Boss Battle: Seymour Flux and Mortiorchis
The Mortibody sure got an upgrade from last time, didn't it? First of all, you're up against Seymour, so aeons are only good for one turn before being banished. Your two opponents still do combo attacks but the main one this time is Lance of Atrophy followed by Full-Life. Lance of Atrophy does about 650 damage but inflicts Zombie as well, most of the time. Full-Life on a Zombified character is the same as Death on a normal one, only with a higher success rate. Most of the time they won't get other attacks in, although Cross Cleave from the Mortiorchis will hurt quite a bit. Seymour's got a lot of random White Magic counterattacks which he'll use according to what hits him - Protect with physical attacks, Shell with magical ones and occasionally Reflect with overdrives. He'll also cast Flare as well from time to time, doing about 3000 to one person (this can be reduced with Shell). Flare is usually preceeded by Dispel on all party members, so watch out. Also, together these two can use Total Annihilation. The name is pretty brutal by itself. This is basically about a 20-second-long stream of attacks to random people at between 500 and 800 damage apiece. Defending doesn't protect against it and it's highly unlikely Protect or Shell will save you either. This is preceeded by "Mortiorchis: ready to annihilate!" when it will use said Total Annihilation on the Mortiorchis' next turn.
Now, how'd you beat it? Seymour has 70,000 HP and the Mortiorchis works in exactly the same way as the Mortibody did in the previous Seymour battle. First you should start with the appropriate party boosts, like Hastega. If Kimahri learnt Mighty Guard in the Biran/Yenke battle earlier on and he has an overdrive here that will speed things up well. Cure Lance of Atrophy's Zombie status as soon as possible because otherwise Mortiorchis will follow it up with Full-Life and kill someone. White Wind also, for some reason, cures Zombie. Aeon Overdrives obviously help here, 5 of them at 9,999 minimum plus the damage Mortiorchis takes healing itself, and you can deal just over 60,000 damage to Seymour. Of course, that's the easy, cheap way to beat Seymour, although since he has Total Annihilation you might not want to take a fairer option to beat him.
Character overdrives help as well, and if you are confronted by Total Annihilation, just summon Aeons to put Seymour and Mortiorchis' turn order out and delay it a little. As for getting rid of Seymour's assisting White Magic, Dispel works a charm. Seymour's random weakness to Bio hasn't changed, but outside of that he's immune to most status effects (including Slow and all four Breaks). The Mortiorchis' Cross Cleave can really hurt if he uses it twice so use Mega-Potions to remedy this. If all else fails use a Mega-Phoenix or even a Megalixir. Make sure to Dispel Seymour regularly. Just make sure everyone gets a turn. If Seymour had more health then this battle would be nigh on impossible.
Mt. Gagazet's Interior
Unfortunately, after two boss battles of sorts and a long and annoying trek up Mt. Gagazet you've still got a really long path through the inside caves of Gagazet until you reach the other side and can begin descending to Zanarkand Ruins. Before you head on past the Save Sphere, check on the left-hand side by the pillars since there is a chest containing the Saturn Crest concealed here. You still can't use these yet so don't worry about it. Save, then follow the arrow to the next area, an immensely-weird part of Gagazet indeed. This is a rather long series of cutscenes in which you'll learn a bit about why Tidus is where he is now, and what he's been put there to do. You can move around in them but there's only ever one place you can go. Once you're back with the rest of your party, follow the ledge around until the music changes and you're inside Mt. Gagazet.
There's another new set of monsters you'll encounter inside here:
- Mandragoras are similar to the Ochus and the like you found earlier on in the game, except these have a lot more health - 31,000 HP. Their physical attack does about 1000 damage and inflicts poison reliably, so make sure you cure it quickly. Ochu Dance is also very annoying, but rarely used.
- Grendels are dual-horn fiends, so they're armoured and quite powerful. Inflict Darkness on them with Dark Attack/Buster from Wakka and then have Auron beat the crap out of it.
- Ahrimans are floating eyeball fiends, so use the appropriate tactics (hit them with Wakka's ball). Watch out for confusion.