Kingdom Hearts HD 1.5 Remix - Final Mix: Ursula, Second Battle

Make sure you have Mermaid Kick equipped - it's essential for this fight.
When Ursula blows bubbles, just circle to dodge. After she tries to inhale you, there will be a brief window for attacking (While she says "Ew, not very tasty.") To dodge her lightning beam, just swim up and over and get a few hits in on the back of her head. Finally, when she charges up her area lightning attack, get the heck out of Dodge by swimming away.