Seymour with guards, Anima, and Seymour again
HP: 6000, 18000, 6000
Difficulty: Hard
Location: Macalania Temple

This battle is actually three fights, but they’re all in one battle. This is
also a hard one, as it took me three tries. I’ll guide you through each
fight below.

Fight 1: Seymour and two Guado Guards
As long as the Guado Guards are around, you won’t be able to damage Seymour.
The other problem is that the guards will ALWAYS heal 1000HP when you attack
them with no matter what. This means that you’ll need to do more than 1000HP
to get rid of them, otherwise they’ll just stay there. When the fight first
begins, have Tidus use the Trigger Command Talk, and talk to Seymour to
raise Tidus’s strength by A LOT. Do the same with Wakka and Yuna to raise
their Magic Defense. After that, summon your best Aeon (you may notice the
Aeon “?” at the very bottom of Yuna’s summon list, but don’t summon her
yet) and try to get that Aeon to Overdrive. If you have a Sensor for your
weapons, look at Seymour’s description. Notice how he uses different
elemental spells in order? Summon Ixion if he’s about to cast Thunder, Ifrit
when he’s about to cast Fire, and etc. Have the Aeon get into Overdrive and
use their Overdrive spell. This should Overkill those damn annoying Guado
Guards and do more than half the HP Seymour has. He will immediately summon
his Aeon, Anima.

Fight 2: Anima
Anima will use only three commands – Pain, which will probably instantly
kill a party member; Boost, which boosts Anima’s overdrive gauge; and
Oblivion, which is Anima’s Overdrive spell that you seriously DO NOT want to
see. If you still have your Aeon from last round, keep them out. Anima’s
Pain spell will only do around 500 damage to them, so you might still be
able to get that Aeon up to Overdrive. If not, get Tidus to attack, Lulu to
use spells, and someone powerful such as Kimahri or Auron to attack. Always
keep an eye on Anima’s Overdrive gauge. If it’s nearly full, get Yuna out
and have her summon her weakest Aeon to take the damage in place of your
main characters. After Anima uses Oblivion, finish him off. He should be
near death. Before killing him, however, have Tidus cast Haste on all of
your characters and heal them up to max, then deliver the finishing blow to
Anima. He’s not that hard, just watch out for his Oblivion spell.

Fight 3: Seymour
He starts with 6000HP at max, and he’s one bad-ass mofo this time. If you
hastened your characters with Tidus, however, you should be able to kill him
before he unleashes his multi-element spells. Make sure to have Tidus, Lulu,
and someone else out, like Kimahri or Auron (haste Lulu during the Anima
fight!) and have Tidus attack, Auron or Kimahri attack, and Lulu use magic.
Seymour’s multi spell is devastating, keeps me wondering how a simple
Thundara/Fira/Blizzara/Watera can do so much damage, but if he is able to
cast it, forget healing. Instead, keep on attacking him. And if you had your
characters hastened, you should be able to defeat Seymour before he casts
his spell again. Remember to keep Tidus out at all times. If you had Tidus
talk to Seymour in the first round of this fight, Tidus’s strength would
risen. By A LOT.

in the temple at Zanarkind

This battle is the reason Yuna and the others become "traitors" for "killing" Seymour. Yuna goes alone to let Seymour know of her answer in marrying him, but she secretly is attempting to kill him, because of the sphere she saw of his father... forgot his name... and then the others join her and fight Seymour together. Ring any bells now?

The 3rd part of the yunalesca battle where she casts mega death right at the start of that part of the battle