Shiva can easily OHKO Anima with Diamond Dust, and her special attack does enough damage that I wouldn't worry about healing her, because she won't be out that long. 3-4 hits max to kill Anima, then Seymour will kill her with one hit.

Edit: Use Tidus to cast Haste on Yuna, and abuse her Nul Shock, Nul Blaze, Nul Frost, and Nul Water. Also, pay attention to Seymour, because he always casts his multi spells in the same order. So you shouldn't be caught of guard by them.