I leveled up my character by having them all in battle.
Seymour isn't really hard if you know FFX system well.

My strategy: (Have all of your aeons in overdrive!)
First, Switch for all characters for AP.
Use Yuna, Tidus and Auron (Wakka is good but I don't think he's good here).
Switch Tidus with Kimahri or Rikku and use Steal or Stone Breath, the Guado Guardian should be gone by now with your Auron attack.
Switch back for Tidus, have him to Haste the party.
If you don't have NulSpell, then you really have problem but don't fear! you don't need that, I use Reflect on Tidus and Auron making them completely protected by magic. Don't use on Yuna, you'll be stuck with healing and gonna keep relying on healing items to heal her.
Attack him with your Tidus and Auron.
Once he summons Anima, Summon Shiva, keep using Heavenly Strike, if Shiva is low of HP, cast Blizzara on herself to heal. Keep attacking and healing until Anima's HP drop to 9999, At that point, you should be boosting and healing, If Anima's Overdrives is full, SHIELD! Shiva should be in overdrives after this attack, use Diamond Dust to eliminate Anima.
If your Shiva died before Anima, have Lulu to keep using Blizzara/Blizzaga, if Anima's Overdrives is full, summon Valefor to give Energy Ray/Blast then your Valefor will get attacked by Oblivion. You should already kill Anima before she can launch Oblivion again.
And Then, Summon Ifrit and Ixion to give Hellfire and Thor's Hammer for overkill.

Done, hope it help.