Very well. I will give you your death. You seem to want it so.
Seymour transforms into Seymour Natus with a mechanical, hovering creature called Mortibody to assist him. Tidus, Yuna, and Auron can speak to Seymour at the start of the battle with Trigger Commands to raise their Strength or Magic Defense.
The Mortibody starts a round by casting a basic elemental spell on the party. Seymour will follow with a Doublecast of that spell's Level 2. Mortibody will regularly use Desperado, which removes all positive statuses.
When his HP drops past 24,000, Seymour will protect himself and start using Break on one character. Mortibody will attack with Shattering Claw, and if the Break status is active and it targets that character, they will shatter and the active party will be down to two members. Below 12,000 HP, Seymour will cast Flare, inflicting as much as 2,500 damage to one character. At this stage, Mortibody will start casting Cura on Seymour which can be reflected back to the player party.
If Mortibody is KO'd, it will regenerate by absorbing HP from Seymour—first 4,000, then 3,000, 2,000, and finally 1,000 HP each time after that.
If an aeon is present on Seymour's turn, instead of his usual attack he will use Banish, which ejects the aeon; unlike instant death, aeons are not immune to Eject. Banish is never used against characters, though if hacked to do so, it functions just like any other ejection attack.
Seymour changes his attack patterns as his HP drops, however he must be attacked to cause the changes. If Poison damage brings him under the required value, his pattern will not change until the next time he is attacked. However, Mortibsorption will trigger the pattern change.
NulElement is useless unless the party has Haste, which can let the NulElements take effect after Mortibody attacks. Reflect is a good alternative as only Mortibody's Shattering Claw can hurt the team then, although Desperado nulls all status enhancements so the party must reapply the status.
Summoning an aeon that has a full Overdrive can inflict massive damage. However, long-term use of an aeon is not possible in this battle due to Banish. The player can fill up the aeons' Overdrives before the battle and then summon them one by one.
Repeatedly killing Mortibody can be used as a way to damage Seymour. This can be exploited further by casting Bio on Seymour, causing further damage.
Equipping SOS Reflect armor results in near-invincibility, although it is very hard to obtain such an armor by this point.
Natus is Latin for "born". This effectively makes the name "Seymour Reborn". He is called "Seymour spectral" in the French version and "Neo Seymour" in the Spanish version.