Now go to the cabin and around the airship alot talking to people. After talking to Cid enough times and Kimahri and Yuna at the part where you saw Evrae, go back to Cid. You can now go all around Spira and get the hidden aeons.
DO NOT GO TO SIN UNTIL YOU FINSHED GETTING AEONS AND YOU CAN DO A LOT OF DAMAGE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Now, visit the Omega Ruins (make sure you are really strong)You should learn Holy, Ultima, Doublecast, Full-Life, Auto-Life, and Quick Attack.
Left Fin and Right Fin HP - 65,000 each
AP - Right Fin: 17,000; Left Fin: 16,000
Weakness - None
Steal - Left Fin: Mega-Potion, Supreme Gem; Right Fin: X-Potion, Shining Gem
Cast Hastega on your party and doublecast flare or ultima on the fin. Summon aeons and use their regular attacks on the Fins. But Yojomo does the best job. Sicks his dog on it. The Fins aren't hard at all so just fight them and cast Curaga and use Al-Bhed Potions alot.
Sin Core and Sinspawn Genais
HP - Sin Core: 36,000; Genais: 1,800
AP - 19,000
Weakness - Genais: Fire
Steal - Star Curtain, Stamina Spring, Supreme Gem
Here summon the Magus Sisters. Try and use Cindy the most for attacking(the middle tall one). Razzia can do up to 80,000 damage right away! Or use Anima to see a pretty good death...
