Ursula is one of the bosses in the world Atlantica. Fighting her is made possible in Kingdom Hearts and Kingdom Hearts: Chain of Memories, but in Kingdom Hearts II the combat is replaced by a song sequence, Ursula's Revenge.
The first encounter with Ursula is in Ursula's Lair. Ursula has stolen King Triton's trident, but does not use it in this battle. Rather, she fights by swinging her hands and tentacles, and every now and then, she will spin vigorously round the room, injuring Sora and company if they come near. This is the most difficult attack to avoid, so ignore everything else when she does so, and concentrate on swimming up and down to avoid her when this happens. Flotsam and Jetsam are also in the lair to deal damage to Sora, however, they are simple to avoid, and Ursula will use cure on them if they are both defeated, so it's easier to ignore them, and concentrate on Ursula.
It should be noted that in Ursula's lair, there is a huge cauldron in the center, filled with color-changing liquid. The color of the cauldron does not indicate what kind of spell needs to be used on it, only what kind of spell Ursula will spawn from it. Launching spells at the cauldron will eventually cause a massive amount of energy to be released, stunning Ursula and her lackeys. Ursula will also throw magic potions to activate the cauldron, however, if there is little to no intervention, Ursula will use spells of her own to damage Sora's party. The easiest spell to cast onto the cauldron is Fire, as it consumes little MP and it is the first on the Magic menu. One may lock on to the cauldron to cast the spell, though it should be noted that physical attacks will not work on the cauldron. Physical attacks will refill Sora's MP, so if running low, make sure to attack Flotsam and Jetsam, as they refill the MP more than other enemies do.
Once Ursula is stunned, take this opportunity to use physical attacks on her. It is also possible to attack her while she is conscious, however her HP will go down slowly, and she will use her spin attack after two or three hits, so it would be better to attack the cauldron. Like all other boss battles, heal when necessary.
Ursula can be found in an area hidden beyond the Calm Depths (marked with "???"), where the water currents might push Sora away—use the newly acquired ability Mermaid Kick to get to that area. In this area, Sora's party will confront a gargantuan Ursula, this time, wielding King Triton's trident. It should be noted that if Sora uses any form of Aero magic here, this will strengthen Ursula. She deals non-elemental damage which resembles Thunder; these attacks increase in power and quantity when she raises the trident. She occasionally blows air currents into Sora, dealing substantial damage to him. When she has only one HP bar left, Ursula can open her mouth to use a Thunder-like laser, which will damage Sora if he gets in the way.
Her only weak point is the head, so it is best to use physical attacks on it. It should be noted that Stop will immobilize her temporarily, making it easier to chain combos. Try and get behind her head, and start hitting her relentlessly. This can be done best by swimming over her head, and then coming down. Occasionally Ursula will push Sora away from her head, so get as close to her as you can. Don't be shy about using the Keyblade—and heal when necessary. When she starts to gasp, swim behind her head and chain combo attacks to the side or back of her head, and swim away when it starts to get dangerous. Magical attacks can damage her as well, but normal combos work best.
If Sora is in front of Ursula when she starts to inhale water, indicated when she opens her mouth very wide, use Mermaid Kick to swim away as fast as possible until she cannot inhale anymore. She will bite Sora if you do not, which can cause a serious amount of damage. She always declares her other attacks—a bubble attack avoidable with Mermaid Kick and raising the trident indicates that she is going to form a circle of damaging magic around her. This can be avoided by swimming away.
The prize for winning is a Thunder upgrade and another Ansem Report.
Kingdom Hearts: Chain of Memories
Sora's encounter with Ursula is on some sort of pedestal on the water surface, and again, Sora fights a gargantuan Ursula.
Note that while Ursula is alert, most sleights do not work on her, so it is best to use melee attacks. It is recommended to stock cards from numbers 7-9, as well as a few Cure cards—and if there anymore space for more cards, fill it with 0 cards in order to break Ursula's attacks. One may consider getting Premium Bonuses for all cards as they reduce the cost points of each card (especially for cards 7, 8, 9 and 0), allowing room for more cards on the deck.
In Chain of Memories, the Gimmick Card, if used, the ground lifts up so that you can attack her face and her face only. In Re:Chain of Memories, it causes Ursula to choke, and be stunned for a period of time. It is then and only then that Sora should use sleights on her. However, the time period is rather short—Sora can only perform seven melee attacks, or two sleights, before she recovers. Be sure to heal when necessary. Alternatively, one may use the Search Ghost enemy card to drain out Ursula's HP—every hit Sora lands restores some HP, or use the Oogie Boogie enemy card to heal tenfold.
This same strategy can be used while playing as Riku in Reverse/Rebirth, only that this time, Riku cannot stock up Cure cards—use either Mickey or Oogie Boogie to recover.
In Kingdom Hearts: Chain of Memories, two tentacles must be taken out to harm Ursula; but in Kingdom Hearts Re:Chain of Memories, it takes all four.
Kingdom Hearts II
The combat sequence is replaced with a musical sequence, Ursula's Revenge. There are only two commands in this sequence—a blue circle with ( in the Japanese version) which must be hit at the right time, and a purple circle with ( in the Japanese version), which must be hit as many times as possible to drain out her HP. Successfully executing both these commands will deplete Ursula's HP.
Sora needs to bring Ursula's HP down to at least ⅛ of the bar to clear this round, though it is recommended to completely deplete the bar.
Kingdom Hearts 3D: Dream Drop Distance
Ursula holds the damaged raft built by Riku and Sora with her tentacles, making it easy to strike her. She blows explosive bubbles and summons lightning strikes at will despite the absence of the trident. At one point, she'll grab Sora, forcing one to wait until Riku hits her until Sora's freed. At some points, she'll lower the raft, using simple attacks. Rely on deck commands, and she'll be easily defeated.