Help with Seymour

[Lea]

I'm fighting with him for the third time, and he literally destroys me.
Any hints? I'm having lots of diffculty.

[ReeNoiP]

My memory is a bit fuzzy. That's the one in the mountains right?
Some tips I can remember on that fight:
He can be poisoned fairly easily. Guaranteed if you use a poison fang. It does a lot of damage over the fight.
His aoe attack later in the fight is magical and can be mitigated with shell.
Cross cleave has the delay effect. Keep that in mind.
Some people like to haste his helper to desync the lance->full life combo so you can heal zombie.
As usual, try to keep your buffs up.

If you can't be bothered you can always cheese it with aeon overdrive spam.

[Back2ZeRo]

"If you can't be bothered you can always cheese it with aeon overdrive spam."

Hahhahahahahhaahhahahahahahah. I missed my childhood =(

[MustaBeenRatz]

Play the game, you ♥♥♥♥!

[TSense]

Poison, then focus on healing and defending.
I also got auto-cure at that point somehow

[ramsza]

A small addition to all that has already been said:
If my memory is correct, then cou can get Yuna's celestial weapon and it's first uprade at this point. It's still not a good weapon with only the first upgrade, but it will give break damage limit to Valefor, making it's overdrive even stronger (if Yuna is properly trained).

[Demosthenes424]

There's actually a specific pattern that their actions follow, depending on Seymour's HP.

Above 35k HP:
Seymour uses Lance of Atrophy, Morti follows up with Full-Life
Seymour uses Lance of Atrophy, Morti follows up with Full-Life again
Seymour uses Dispel, Morti follows up with Cross Cleave
*loops back to the start of this phase*

When Seymour first drops below 52.5k HP, he'll cast Protect (only once), then continue looping.
When Seymour first drops below 35k HP, he'll cast Reflect and Mortiorchis will say "Auto-Attack mode"

Below 35k HP:
Seymour casts Flare on himself, Mortiorchis says "Ready to Annihilate"
Seymour says "Watches... and waits" if he is still Reflected; if Reflect has been dispelled, he recasts it instead.
Mortiorchis uses Total Annihilation.
*loops back to the start of this phase*

A slight quirk to this: if Seymour is poisoned, or you wear him down by repeatedly killing Mortiorchis and letting it drain his HP rather than directly attacking Seymour, they'll be stuck in the first phase until you hit Seymour directly. If you find the LoA/Full-Life combo much easier, you can try it this way. I just used Aeon overdrives from about 40k HP, which also interrupts their actions by forcing him to Banish instead of his normal action; Mortiorchis won't use Total Annihilation until Seymour continues the normal pattern, but before then you can just summon another Aeon and blast him with an overdrive. Another advantage to this is that you'll easily overkill, for two more L4 key spheres.

Hands down the easiest way to beat Seymour, or really any of the normal storyline bosses from Calm Lands on, is to use Rikku to Mix Trio of 9999 using two Wings of Discovery from the Remiem Temple race. It makes all actions do 9999 for the current party. A few multi-hit overdrives later, down goes the boss; Yunalesca is the only one it gets complicated for, since if you use Trio and kill phase two without letting her use Hellbiter first then she'll immediately open phase three with Mega Death and kill all your non-zombie characters.

[Chillin]

holy water is your best friend

[solidap]

It's a pretty tough fight if you don't have Hastega. He's also susceptible to Slow. So Haste + Slow gives you a huge advantage. I'd suggest grinding in the snow area to unlock those nodes for Tidus

[HandOfKane]

What I usually do is steal a lot of those elemental gems from enemies (the items that cast 5 -ga spells of their respective element at random enemies for about 600 damage each spell), and then throw them at him at every opportunity you can. You can steal lots of Fire Gems from the Grenade enemies in the area. I also moved Kimahri into Rikku's area on the Sphere Grid so he could learn the Use command too. Often times the gems will kill Mortiorchis too (especially once it's at 1,000 max HP) which will make it drain HP from Seymour.

Make sure to keep Haste on your characters so you get a chance to cure Zombie before you get hit with Full-Life and to dispel Seymour's Reflect (which will make him cast Flare on himself). Also use Shell to help survive Total Annihilation and poison Seymour (with Bio or a Poison Fang) at the start of the battle.