Final Fantasy X/Aeons

Anima is Seymour's aeon. You can see it in a scene after the blitzball game in Luca, and you fight it in Macalania. However, you can also get this powerful aeon to fight for you.

Obtaining Anima
You can obtain Anima by destroying the Destruction Sigil in each Cloister of Trials, then going to Baaj Temple, which can be found with the right coordinates. Once there, dive into the water from the rocks north of you, then swim towards the entrance of the temple to find...

Geosgano
HP: About 32.000 Sensor: Vulnerable to magic. Immune to: All negative effects, except for Poison. However, there is another way to inflict negative status on him... Attacks: Stone Punch - Physical damage and Petrification to one character. KO Punch - Physical damage and Death to one character. Eat - Will eat one character. Will copy ALL status effects, including positive ones. Will later spit character out, inflicting damage to all characters equal to the amount of HP the character had when he/she was eaten. If you use Struggle inside his belly, he will spit you out immediately meaning instant KO, I do not know if this means damage to the party too. If you kill him with someone in his belly, he will spit the character out inflicting 9999 damage to the party.

Strategy
This is the fiend that almost had Tidus for lunch in the beginning of the game, like Tidus said: "Payback time!" Death and Stoneproof armour is needed, trust me. At least use Stone Ward, because if you are petrified underwater, you will shatter and won't come back for the rest of the battle. First start out by inflicting Poison on Geosgano. I mixed up an Aqua Toxin with Rikku (Candle of Life, Fish Scale), because no one could use Bio. Then start inflicting status effects on one of your own characters. If one of your characters knows Cura(ga), inflict Zombie. I'd also recommend Slow. If Geosgano is zombified, cast Cura(ga)'s on him until he dies. Be sure not to kill him with a character in his belly! This will probably mean instant KO, which means you'll get Game Over. Good luck!

Attacks
Her special attack is "Pain". This attack inflicts damage and Death to the opponent. The first time you see Anima is after the Blitzball game that you get to play at Luca when all the fiends start attacking. That is also the first time you see "Pain".

Her Overdrive is "Oblivion". Oblivion is when Anima takes you into oblivion (hence the name). Inside Oblivion, she breaks the chains that ties her hands down. It delivers hit after hit. This means that you don't need Break Damage Limit to inflict a lot of damage with this Overdrive.

When you first get her, she comes with all elemental Black Magic spells, plus Bio and Death. She also comes with a lot of status inflicting attacks, including Zombie Attack.