You know- this boss battle was the most difficult for me, too. It took me a couple tries to get the hang of it. Just blast the cauldron with magic spells and when she's stunned, hit her. Recharge by hitting Flotsam and Jetsam. I hated the Ursula battles... they sucked... The only other boss that gave me trouble was Riku in Hollow Bastion (the 2nd time you fight him).