TO defeat Ursula u must attack her potions pot a lot . u attack it with magic only and it depends wut spell u use. if she pours in a red potion into the pot u use fira , blue potion u use blizzara , and yellow potion use thunder. if u do this right the potion inside the pot will turn white and then u can attack ursula until the pot stops glowing white and then u have to do the process all over again until u defeat her. if u dont do this process ur attacks will only do one damage and that would take a while. oh yeah , and dont attack her fish friends who swim around and attack u. its just a waste of time since when u kill them they come right back to life. i hope i helped!
from eric krohn the master of kingdom hearts

oh yeah! i completely forgot about ursulas second form. to get to this form use ur new ability mermaid kick(that u got from defeating the first form) and use it to go into the room marked ??? (thats the name of the room in case u think i got confused). here u fight ursulas head. this is the harder of the 2 forms. in this battle replace donald in ur party with ariel since he'll try to use all his magic in the first few seconds which is worthless. this ursula has 2 main attacks. the first is an attack where she'll try to bite u or eat u or wutever she does. the second attack is a thunder magic attack. in case u didnt know thunder never misses so try to keep ur Aero up most of the battle to lessen the dammage. to defeat her swim around to the back of her head and attack her neck (this is the only way to hurt her). she'll try to shake u away from her head and it might take a while to get a few good combos in. do this until u defeat her.i hope i helped!
from eric krohn the master of kingdom hearts