Boss Battle: Seymour
This battle has a few difficult features to it; Seymour is no less potent than he was when you used him to defeat Sinspawn Gui. He has 6000 HP, whereas his Guards have 2000. The battle will begin with the 2 Guards casting Protect on themselves and Seymour casting Shell on himself, which halves physical attacks and magical attacks respectively. Change your party to Wakka, Tidus, and Yuna and use their "Talk" trigger commands to double Tidus' Attack and double Wakka and Yuna's Magic Defense. Next cast Haste with Tidus on Yuna, then switch Tidus out for Rikku and Wakka out for Auron.
Someone should have a weapon with Sensor equipped — if not, use the Weapon change option to do so. This way, the handy Sensor information bar will tell you that Seymour has a nice predictable attack patter which goes ice, thunder, water, fire. So, you should make note of this order and have Yuna repeatedly cast the appropriate Nul spells so that Seymour cannot inflict any damage on you. However, any non-magical attack against Seymour will be blocked by his Guards, and if you attack them, they will heal themselves, faster than you can kill them. They also have a Shremedy attack, which inflicts confusion on one character (you have to hit them to knock them out of it, or use one of your precious Remedies). Don't worry though; this is why Rikku is out. Have her Steal from both the Guado Guards to disable their healing abilities, then just obliterate them with magic from Lulu (switch Rikku out for her once you've stolen from both of them, but you can also nab a Turbo Ether off Seymour). Once both Guards are dead, pound Seymour with attacks. When he gets to low health, he will summon his Aeon, Anima, to deal with you. It is important however that you do NOT use any summons for this stage of the battle and that all of the characters that will reappear after the next part have all four Nul spells on them and Yuna is in Haste. Do not kill Seymour until this is the case, otherwise once you reach the end of this chain of battles you will die.
Boss Battle: Anima
Anima looks tough, and most importantly, is tough. The only attack it will use against you until Overdrive is Pain, which inflicts a good amount of damage but most importantly, instant death. Fighting Anima with just your normal party will end up with you being systematically killed faster than you can revive yourselves. So, the absolute first thing you should do is summon your new Aeon (named ?????). Aeons are immune to instant death and so she'll be able to take Anima's Pain without too much trouble. Use Blizzara on her when her HP gets low to replenish it, and keep attacking normally and with Heavenly Strike until her overdrive is full (it'll charge much faster than Anima's). Then, use Diamond Dust for a lot of damage. Your Aeon should be doing at least 8000 damage with Diamond Dust; if she isn't, then level up Yuna more at the next earliest opportunity and use the Magic Sphere you have (if you haven't already) on Yuna as any improvement to her stats will boost all of her Aeon's stats in the same area. The fact that Anima alternates between using Boost and Pain helps to keep your new Aeon alive during the turns that she loses from using Diamond Dust. The only problem is that Anima must die before its overdrive is full because Oblivion will do a large amount of damage but if your aeons are strong enough then shelding should stop your aeons from dying. Inflict the 18000 HP damage you need to kill it, then Seymour will enter the fray once more (and likely one-hit your new Aeon with an -aga level black magic elemental spell).
Boss Battle: Seymour…Again
Now Seymour will be back at full health (6000 HP) and you must fight him again. The only differences this time are that he has no guards (even if you didn't kill them the first time) and uses 4 new deadly spells called Multi-Fira, Multi-Blizzara, Multi-Thundara, and Multi-Watera, which involve him casting said spell 2-3 times in a row, hitting multiple characters and doing a whopping 800-1000 damage with each. But you were a good reader and casted all those Nul-spells before, didn't you? Keep recasting them and Seymour will hopefully never land a hit on you. As a fail-safe you can also hit Seymour with Magic Break, which he succumbs to some of the time (it may not always work even though it won't show him as being immune to it). Also to really hit him home you can use your spare moves with Yuna to heal your party should they sustain damage, or better yet, run across and smack him in the face with her staff for an excellent 50 damage. After you kill him he will fall to the ground and be carried out by his Guado Guards.
