This is just going to tell you how you get to fight her, and hw to beat her.
Ursula is around the ship wreck, there is a cavern down in the lower part of this area, go inside the cavern/cave and find your way there, (i can't remember if this is here yet, but if there is a like water bit, where all the water is fast flow at your direcrtion, or your swept in at the current, go to the left exit as the other exit will come in later on) Go inside the cave an follow it round. Now when inside there are 2 exits to this part aswell. There is a cavern nook, which is the save point, or there is the exit with a face like image on the wall. Follow the exit with the face like image to get to Ursula. Now follow this round and you will enter the battle field.
Ursula pt 1
There will be an opening cutscene to this battle so just watch the 30 second scene then begin the figth. In the centre of the battle field is a pot with a different coloured liquid inside, do the following
Red = fire = cast fire on it
Blue = blizzard = cast blizzard on it
Green = Cure = cast cure on it
Yellow = thunder = cast thunder on it
Do this until the screen flashes. Then attack Ursula, you should be able to get this done quickly.
Ursula pt 2.
After the battle is over by repeating that, head out of the lair, back to where you got in through the rushing current. Now Ariel will give you the shared ability of mermaid kick! NOTE: YOU MUST EQUIP IT! IT DOES NOT EQUIP ITSELF
Once this is fdone, use the mermaid kick by pressing O to go in the other exit oposite the one you are in now.
Now Ursula will be HUGE!! so go behind her neck and attack, keep swwimming around and pressing triangle, to get donal and goofy/ ariel to attack. The battle shouldn't be to hard
Hope this helped