Kingdom Hearts: Chain of Memories/Sora/Atlantica

When Sora, Donald, and Goofy arrive at Atlantica, they realize that they're underwater, but strangely have the ability to breathe and walk as if they were on-ground. After the group discussion, Ariel comes along and tells them that, if anybody asks, they didn't see her. The group is confused with her behavior, but they do take notice of something sparkly on her neck. After they take notice of this, she swims away and you are given the Key of Beginnings.

You'll start off by meeting familiar Heartless from the first Kingdom Hearts' Atlantica, like the Aquatank and the Screwdriver, along with some Heartless that wouldn't be expected under the sea like the Darkball, which was battled at Hollow Bastion and The End of the World. After finishing off these groups of Heartless, go to the door and strike it with the Keyblade. Synthesize with a card that has a value of 1 or more to pass through the door and head to the Room 2. The next room will hold more Heartless and another door at the south side of the room. Strike it and synthesize, this time with a card that has a value of 7 or more, and you'll be in Room 3. After destroying all the Heartless here, strike the door to the west and use an Attack Card along with the Key of Beginnings. The next room will hold a cutscene where the three meet Sebastian. Sebastian informs them that the King's Trident has gone missing. All of the clues point to Ariel. Apparently, she's planning to give it to the sea witch Ursula. With this sign of trouble on their hands, Sora, Donald, and Goofy set out to find Ariel before it's too late.

After the cutscene, it's back to battling Heartless. Go to the door on the left and use a card with a value of 5 or more to pass through to Room 5. Do the same for this room - kill all of the Heartless, take the door to the left, and synthesize with a card having a value of 5 or more to get to the next room - in this case, Room 6. After defeating all of the Heartless in this room, had back where you came to Room 5. This time, take the door to the south of the room and synthesize with either a Moment's Reprieve or a card with a value of 5 or more. After battling all of the Heartless, save and go back to Room 5 yet again. This time, go to the door to the north of the room and use a Calm Bounty if you have one. Get rid of all of the Heartless in this room and return to Room 5. Go back to Room 4 and from there, head to the door to the east. You should be back in Room 2. Go to the door at the north of the room and synthesize using a card with a value of 7 or higher. You'll end up in Room 9, where you'll see one door. Strike it and use the Key of Guidance and a card with a green border. You'll be greeted by another cutscene. Ariel and Ursula are having a discussion. Once Ursula leaves, the three intervene and offer their help. After the scene, you obtain the Key of Truth. Now go back to Room 6 and use the Key of Truth along with a Magic Card to get through the door. Another cutscene follows - Ursula obtains the trident and grows much larger. It's time to battle.

Ursula
Ursula, in a mimicking of events from Kingdom Hearts, grows into a much larger version of herself. This boss battle, unlike many of the others, shouldn't be very hard at all. Her attacks aren't very damaging but easy to dodge or card break anyway, and she doesn't move around much either. It'll be easy to stay on top of her by moving around a lot, which she quite obviously isn't capable. Card breaking is a breeze in this battle and even then, dodging also proves quite effective. As long as you're at the right level, which should be somewhere over 25, this boss will be a breath of fresh air. Ursula's attacks include: Once you defeat her and obtain the card she leaves behind, go to the next room and leave Atlantica.
 * Quite simply, Ursula casts Thunder. This acts the same as any other Thunder attack, and can be avoided easily by card breaking.
 * Bubbles come for you and, along with taking away your health, will also confuse Sora. The controls will be messed around with and you'll have to work your way back to Ursula with your new control scheme before the confusion wears off.
 * A lightning strike is emitted from Ursula's mouth, so keep away when you see her preparing this one.