There's a merchant not long before the boss on Mt. Gagazet. If you've already reached the save point, you'll need to backtrack to find him. He sells two "Blessed" items - one for Yuna and one for Auron. I had enough gil to buy both, and I recommend you do too. If you're far short on gil, you might be OK buying just the one for Yuna, but I'd say it's not ideal.

Blessed armor gives a chance to prevent zombification. Seymour's Lance of Atrophy turns a party member into a zombie, and immediately after he casts "Full-Life" on that member. Being a zombie white magic has the opposite effect, so it instantly kills the party member (you can't heal them either). So, the Blessed armor is worth it to prevent death and spending precious turns reviving and healing.

This is perhaps most important, and quick enough to do with a few random battles around the save point. Seymour Flux uses an attack to instantly kill each aeon, but not before they get in a single attack first. You'll need the aeon's attack to be an Overdrive, in order to maximize impact. These attacks will do approx. 10-13K damage (this includes after the attack, when Seymour uses a spell to drain his own health in order to heal his appendage).

Bio will drain about 1-2K per turn from Seymour, so you'll want to cast it ASAP to get its full effect. However, if you have a Poison Fang, you can replace Lulu with Rikku (only Rikku can use this item). After this is done, you'll want to replace her with Wakka or Auron, since Lulu has relatively low HP and Seymour has some devastating attacks (more on those later).

After Lulu casts Bio, Yuna and Auron will be your most important party members. Yuna's summons will do devastating damage. Auron will have enough HP to withstand Cross Cleave, which does about 2K damage to the whole party. This will be the most tenuous moment in the battle, and it's important enough to have at least character with enough HP to withstand it. And both Yuna and Auron have Blessed armor.

Cast Haste on Yuna whenver and with whoever you can. My Auron had Haste, but I'm not sure that's typical. You may have to bring Tidus in for it, even though he doesn't have any other particular strengths.

Summon with Yuna whenever you have a window of opportunity. It will be your most powerful attack.

Wakka's key for his Silence Attack and Silence Buster, and for the fact that he does pretty good damage with those attacks in the process. Seymour likes to cast Dispel, which will void your efforts to use Haste. But if he's silenced, then he can't cast it.

Seymour has a devastating 1-2 punch. He uses Cross Cleave for about 2K damage to ALL party members, then uses Lance of Atrophy to zombify, then uses Full-Life to kill remaining characters if zombification was successful (Blessed armor has a chance to prevent).

The more HP you have ready, the better your chances not to get wiped out. Auron is good for this, as his HP is very high. Don't be afraid to use X-Potions to fully restore his health, if you have them.

Seymour's attack "Total Annihilation" will do about 4K damage on your party members. Yep, you read that right.

Luckily you'll have a few turns' warning as he charges. Use a character with a sensor weapon to gauge his HP, and if you're pretty close then give him all you've got. If you're not, then focus on the party member with the most HP (should be Auron). Heal him up all the way, and cast Protect, Cheer, etc - any skills that raise defense.