You’ll start with only Yuna on your team, and since at this point in the game she probably doesn’t have any offensive spells you must use your Aeons to defeat any monsters you encounter. Use the map provided to navigate your way through the Via Purifico and grab all the extra items as well:
After your done go north to exit the Via Purifico, following the red arrow. At the end you will meet up with Isaaru, who will force you to fight him because he has been ordered to destroy the traitors. He will challenge Lady Yuna alone, and you must fight him with an Aeon of your choice. Might I suggest Bahamut? Watch out though, if your Bahamut isn’t very strong he may not survive Isaaru’s Aeon’s first attack, which is an overdrive. Grothica will use Hellfire, as its first move, doing up to 2000 depending on which Aeon you’re using. His second Aeon, Pterya, is just like Valefor, and his third summon is Spathi, which is much like Bahamut. Spathi will countdown to use Mega Flare, so hurry up and destroy it before it gets a chance to. After you defeat him Yuna, Kimahri, Auron and Lulu will all escape, and you’ll move back to controlling Tidus, Rikku and Wakka.
Via Purifico Underwater
Start by saving at the save sphere just off to the left. There’s a chest on the opposite side of the corridor that you don’t actually get anything from, instead you buy items from it. Swim down the corridor and there will be one more save sphere before you enter the larger room up ahead.
Evrae Atlanta: This boss is super super super easy as long as you know a little something about it. First of all, the regular strategy that I used the first time was to unlock all the locks and by doing so enable you to fight the Evrae Atlanta from a distance once you go through two doors. Now, however, I know a little bit more about how the game works. You must first notice that Evrae Atlanta has the Zombie Status effect. What does that mean? Curative items will inflict damage (it does the same to you, which really sucks). Example, Potion does 200 damage, Hi-Potion does 1000 damage, and normally, Phoenix Down would instant kill. The Evrae Atlanta cannot be instantly killed, though, as that would make it too easy. The Phoenix Down’s instead only do 8192 damage. Who are we kidding? It may not be instant kill, but that’s a hell of a lot of damage! In fact, with the Evrae Atlanta’s HP being only 16000 it only takes two Phoenix Down’s to kill it. Lots of AP when you kill it too, so make sure that your entire party gets it.
If you defeat it without using locks you get two Black Magic Spheres at the end of the battle as well as a sword for Tidus. Continue swimming down the corridor once you’re done.
After your team meets up again Seymour, who will have killed Kinoc, will approach you. He will then challenge you, and morph into a far more powerful being. Although you and your team plan on leaving, all turn around to go get Kimahri. Save at the save sphere before going up and challenging Seymour.