|Aqua Flan||Weak to Lightning|
|Bat Eye||Weak to Fire|
|Cave Iguion||Weak to Ice|
|Maze Larva||Weak to Lightning|
|Sahagin||Weak to Lightning|
|Swamp Mafdet||Weak to Lightning, Ice|
After all that in the Cloister, there will be some scenes in which only the expected occurs. (Well, other than executions, sadly.) You'll be taken into the Via Purifico and control initially just Yuna. Yuna is now in control of the most powerful non-optional Aeon in the game, Bahamut - while not truly the strongest (that accolade goes to the Magus Sisters and Anima) he can break the damage limit of 9,999 from the word go, so, yeah...
Anyhow, when you regain control, save the game. Be careful in the random encounters here; you don't want Yuna to get too hurt, so she'll be relying on running or Aeons in those, and those Aeons are going to become CRUCIAL in about ten minutes. Go east at the fork and, then go north there to find Kimahri . There is also a Mega-Potion chest here. After grabbing it, go north along the path to find Auron and a Save Sphere. Go west from there, then south at the fork. Once you eventually reach the open area, go along the small hall to the east to find Lulu . Also open the chest behind her for a White Magic Sphere, then go to the southwest corner for an Elixir .
Examine the glyph amongst the rubble to warp, then go north and east at the fork to find the start again. Save if you want. Go north now past the glyph in the ground, then go down a hall to the east to find a Black Magic Sphere . Step on the glyph to move the warp panel and then go west, north, and west to find a warp glyph with a moving pointer. When it points west, step on it to find a Lucid Ring and a Skill Sphere . Go east to find a 10,000-Gil chest, then warp back. Go east at the fork to the Save Sphere. Save, then go north to find Isaaru.
And he ain't happy.
BOSS - Isaaru and His Aeons
Isaaru will fight using only his Aeons, and he will only use three: his Ifrit, Valefor, and Bahamut, named Grothia, Pterya, and Spathi, respectively. Only one will be used at a time, and their offenses can be expected. Grothia can use a basic physical, Meteor Strike (delays), Fira , and Hellfire for an overdrive. Pterya can use a basic physical, Sonic Wings (delay), and Energy Ray for its overdrive. Finally, Spathi just uses Mega Flare - it's overdrive - every five turns. Grothia's and Pterya's attacks are pretty expected in their damage range, usually 500~1,250 - it's dependent on your Aeons' stats, Yuna's stats (trust me on that), elemental affinities, and the like.
ALSO, TAKE EXTREME CARE THAT YUNA CAN BE HIT IN THIS BOSS FIGHT! Yes, Yuna can be hit by Isaaru's Aeons, and that is brutal on her if you followed typical tradition on your Sphere Grid (i.e. made her a healer). This is mostly when an Aeon is killed and the opponent is fast enough to get in a turn before Yuna can re-Summon. It's nasty, to say the least, especially if it's an overdrive. And, since she's alone in this fight ... Pac-Man dying noise
Anyhow, Isaaru opens with Grothia, which will be the easiest of the three. It tends to arrive with a full Overdrive, so expect Hellfire on the first turn. If you have NulBlaze on anyone, great, use them and it. Otherwise, DON'T bring in Shiva and simply Shield, then attack: your offense ought to be mostly just "-ra" magic or Attack. This Aeon (probably Valefor) will likely die, so bring in Shiva to counter to settle the rest.
Next comes Pterya, a rather cool name for Valefor. Don't waste any Overdrives here, but bringing in someone other than Shiva is a good idea. Bahamut works nicely; besides, you can't use this mighty Aeon in the next fight, so why not get in some practice, eh? Not much to say in the way of offense, though - blast 'em with all you got, Shield before Overdrives, etc.
Spathi is the nasty one of the bunch. All it will do is sit there and wait until its Overdrive gauge is full in five turns, then blast you with Mega Flare for well over 5,000 damage, which will likely one-hit your Aeons. Counter by bringing in anyone with an Overdrive, but don't use it immediately; rather, keep up a simple offense of Attack or a "-ra" spell. (The delay-inducing attacks slow you down slightly, too, and since Bahamut is immune to Delay, don't bother.) Use Overdrives as soon as possible, though, and be sure to Shield before Mega Flare - you might survive! If you do, heal up and keep battering Spathi.
It's mostly the kind of boss battle where you have to come in wielding Overdrives on all your Aeons, and maybe even a Grand Summon from Yuna. ... That, or some way to induce Doom (kills in five turns here), but that's unlikely to happen anytime soon.
That's basically it before you get control of Tidus.