Under the Sea
After the initial event you’ll acquire the Key of Beginnings . You’ll start to obtain Key to Reward cards from this floor up so collect them and use them immediately (you can only hold one at a time) so you can reap the rewards. If you’ve completed 358/2 Days you’ll get an added bonus! Don’t worry about getting enough cards to get all of them, you’ll get plenty as you play, just remember to go back to previous worlds to get the rewards.
Make your way to the Room of Beginnings to acquire the Key of Guidance . Try to get some Crabclaw cards since it’s a well-rounded Keyblade. When you are ready, take the Key of Truth that you get from the Room of Guidance and go to far left of the map to the Room of Truth where you’ll find Ursula.
BOSS - Ursula
Ursula has tentacles that guard her head so you’ll have to take them out. Destroying the tentacles can also result in a Gimmick Card which lets you get to Ursula’s face, so get to breaking those tentacles!
You’ll want a deck of 9 or higher card, or you need Jafar so you can combo without getting card breaks. Use your Friend cards to break through the tentacles. If you get some down but acquire a Gimmick Card, hold it. Bust up Ursula and when she puts the tentacles back, use the Gimmick Card to get a second round of damage in. You’ll want to break her Thunder attack, but you can dodge roll the others. She drops Urusla Enemy Card .
After the battle make sure you save because you’ll have another boss fight immediately.
BOSS - Riku
Riku doesn’t use any sleights, so that’s a plus, but he does use three-hit combos that have a chance to stun you. You can counter after his jump slash or focus on card breaks. Riku will use a Shadow Enemy Card once he reaches 50% health. You can ride it out or use Jafar at this point so you don’t get broken while trying to fend off Riku.
If you get low you can back off and chances are Riku will just sit back and let you heal. Use Sonic Blade and Strike Raid sleights if you’re having a tough time as they do a lot of damage and have action presses for follow up damage. Riku drops Aero .
Head to the next floor after you’ve saved, but make sure you use any Key to Rewards that you have on previous floors first.