Take Tidus' advice and summon Shiva. Defend against Anima's Pain and heal using the Ice spells until Shiva has a full Overdrive gauge. Diamond Dust will potentially do near max damage to Anima. From then, keep Shiva alive until Anima unleashes Oblivion and then just attack until she is dismissed by Seymour. If Yuna can use Grand Summon and call Shiva with a full overdrive gauge at the start, this part of the battle will go much quicker.