Seymour is flanked by two of his Guado Guardians; their first action is to cast Protect on themselves, while Seymour casts Shell on himself. The Guards will cover Seymour from physical attacks, and due to their Protect status the party cannot inflict much damage on them with physical attacks. They cannot cover magic, but Seymour will not take much damage from magic due to his Shell status. The Guards will use Hi-Potions whenever they are hit with a physical attack. They will also throw Confuse-inducing Shremedies at the party.
After his HP drops to 3,000, Seymour will summon his aeon, Anima. After Anima is defeated, Seymour will have returned to full health and his Magic power will be raised. Seymour will vary his spells which he casts twice in one turn. He will use Blizzaga, Thundaga, Waterga and Firaga on any aeons still standing, but may heal the aeon if it absorbs the element of Seymour's spell.
Seymour's Requiem Overdrive, which he had as a playable character in the battle against Sinspawn Gui, is not used by Seymour, nor is it implied he has access to it.
Using Dispel or Ixion's Aerospark might counteract Seymour's Shell status. The Guards will use Hi-Potions whenever they are hit with a physical attack, but using Rikku to steal these will prevent this action. Stone Breath from Kimahri will kill them instantly. Threaten is also an effective way to deal with the guardians, since they are unable to act while its effect is on.
Using Yuna's Nul spells or Reflect will help negate damage from Seymour's elemental spells. Using Auron's Magic Break will reduce the effect of his spells slightly.
Seymour's increased magic power after dismissing Anima actually comes as the result of not one, but two factors. The first is a direct increase in his magic stat from 25 to 32; the effect of this can be seen on both his -ra and -ga spells. The second is that the Multi versions of his spells are inherently around 60% more powerful than his single versions. The combined effect of these results in the second-phase spells being over 140% more powerful. The -ga spells, which are affected by the increased magic stat but are not inherently more powerful in the second phase, only increase in damage by about 45%.
Interestingly though, although it has no impact on gameplay due to characters being unable to have armored status, the multi versions of the spells are not considered Piercing attacks. This is extremely rare for non-physical damage.