Natus. Fun. If you need more levels, or desire Reflect, level on the Mechs on the Highbridge. Unfortunately, Kimahri will miss out. Sucks for him.

There are two ways to go about the battle. Aeon Overdrives, or no Aeons. I suggest the first. But also make sure all characters get Experience by having them do at least one action in a battle. Attack Seymour if you attack, not the Mortibody if you go with the Aeons technique.

If you only use Aeons, make sure all of your Aeons have Overdrive ready, and that this is the first thing you do in the battle, as it can cause the most damage. Switch the party until it is your fastest characters (most likely it will be Yuna, Tidus, Rikku; yes Yuna is the second fastest character until her Sphere Section is done. Cast Haste on Yuna, and have Rikku Steal, or use a Lunar Curtain on Yuna. If someone happens to be petrified already, use an Al Bhed Potion or a Soft.

With this setup all ready, summon all your Aeons. Most of the Aeons should be able to do 9999 damage, and Bahamut should be able to do more than 9999. Use Bahamut first. He should do anywhere above 11000 damage, and add that to the fact that the Mortibody will drain Seymour of HP when its HP is depleted. This amounts to a damage of 15000 or more. After this, use any other Aeon you want, and unleash an Overdrive. This should do 9999+3000, then 9999+2000, and lastly, it will do 9999+1000 from any Aeon after the Mortibody drains only 1k.


Alternatively, you can fight head-on. Seymour will unleash very powerful spells, including some that you will never hope you can use until late game. You have 2 defining points here: Yuna with Reflect, and Yuna without Reflect. I personally have NEVER had Reflect while fighting Seymour Natus (I never got it until Mt. Gagazet).

If you have Reflect, cast it on your characters, as well as Seymour. Seymour may be able to cast spells, be he is still dumb enough to have his Mortibody attempt to heal him directly instead of detecting Reflect and using it on you instead. He is also dumb enough to cast spells on you without checking for Reflect, which means he will cast Flare and suffer 9999 damage.

If you do not have Reflect, you will have to do this the old fashioned way. You will have to deal with level two elemental spells, in the same order as that of the regular Seymour battle. If you have Sensor, it should give you the order. If not, find the order. Nuke your Nul-Spells as fast as you can, supported by Haste on Yuna. Use Shell for good measure if you happen to have the time. Have Tidus, Auron, Wakka, and Kimahri attack either the Mortibody, or Seymour. Be aware of the Mortibody's HP; if you use Auron (or any other character) and he does 2k damage, then make him attack Seymour, instead of wasting that 1k damage. Lulu should use spells, and Rikku should either Mix, or use her items that cause damage.

Seymour himself will use a plethora of spells. Level 2 elemental spells in an order, Break, and Flare. The first 4 spells can be stopped, depsite the fact that he casts two of them at once. Break does nothing except Petrify (no damage). However, the major danger is that if the Mortibody gets a turn right after Seymour, it will use an attack that hits all three characters, permanently removing the Petrified Character, and slightly hurting the other two. This can be delayed if you deplete the Mortibody's HP, causing it to drain HP from Seymour, and delaying its turn. However, Seymour can still kill the character by his spells. Cure with Soft or Al Bhed Potion.

If Seymour uses Flare, there's nothing you can do without Reflect. Just revive the character after all is said and done. At certain parts during the battle, Seymour will counter all attacks made with either Flare, or a random spell. So finish him as fast as you can. If you want to, bring in Bahamut as well, and you might be able to score an Overkill.