The order in which he casts (aforementioned) is very useful because it's the same order he casts it in two future battles.

Yuna's nul-spells work nicely... also, if Kimahri doesn't have stone breath, just use Rikku to steal from the Guado Guardians and kill them first... then concentrate on Seymour.

With Anima, depending on how strong your aeons are vs how strong you are, I used my aeons as shields, basically, summoned either Shiva or Ifrit, because they can heal themselves, and let them take the blow of Oblivion. Using shield cuts Anima's overdrive down considerably.

If you have trouble with this battle, but still manage to defeat Seymour, my only suggestion to you is this: level up hard and fast before Mt. Gagazet.