Stuck on first Seymour fight

This is my first time playing FFX and I am stuck on the first Seymour fight, so far I've only tried the fight once, but I threw everything I had at the first 2 phases, especially Anima, and then Seymour absolutely wrecked me on the 3rd phase. While fighting Anima, I summoned Shiva, and her Overdrive only did about 4,500 HP of damage, while my cousin did the same attack on Anima and it did 9,999. But I still beat Anima. How can I avoid getting wrecked by the 3rd phase?