Have everyone talk to Seymour to gain some stat-ups. To defeat Seymour's two guards, have Rikku steal their Hi-Potions, then kill them. Cast Haste on your characters if you have it, and have Yuna cast NulBlaze, NulFrost, NulShock and NulTide on your party. Attack Seymour for a while, and keep recasting the Nul-spells if necessary. Eventually, he'll summon Anima.

Using the new Aeon, Shiva, should make Anima really easy, although I've owned Anima even with Valefor. Just have Shiva attack her and occasionally maybe use Blizzara on herself for healing. If Anima uses Oblivion, have Shiva defend, she should be able to survive it. Diamond Dust should make short work of Anima.

Seymour will start to spam his elemental spells, so hopefully you cast all those Nul-spells. Keep recasting them with Yuna while your other characters attack, and he should eventually go down.