You’ll need plenty of cards to get through this area, especially to enter the Room of Rewards. Destiny Islands is pretty small compared to other worlds so you might need to recreate rooms if you can get the right stuff. You get the Key of Beginnings after the opening scene. This is another great spot to level up if you feel weak, so take your time through this world. If you can, try to get into the Room of Rewards for the Megalixir (and Two Become One if you’ve completed 358/2 Days), it’s really handy.
In the Room of Beginnings you’ll acquire the Key of Guidance . Navigate to the end of the world and enter the Room of Guidance where you’ll find the Darkside waiting.
BOSS - Darkside
Darkside is one of the few bosses in the game where you want to avoid the use of sleights. Use a bunch of high level cards so you can break all of the Darksides attacks. The ones that can cause some trouble are when he fires balls of darkness from his chest and when he breaks a ball of darkness to rain it down upon you. These are easy to break because they aren’t sleights. Focus on breaking these and jumping over his slam attacks. The Gimmick Card in this fight will create pillars for you to reach Darksides head, making it easy to get a lot of hits off.
After the battle you receive the Oathkeeper . Be sure to save in the Conqueror’s Respite because Riku is waiting for you again.
BOSS - Riku
Riku is much stronger now and has the powerful Dark Aura sleight. It is one of the most deadly sleights in the game, so it is imperative that you card break it or you’ll likely die. If there is a single attack to focus on in this battle it’s going to be that one. Equip the Vexen and Jafar-Genie Enemy Cards so you can go all out early. When Dark Aura starts to get used repeatedly, you can use Vexen in case you don’t block it. You’ll want to use a lot of sleights and have quite a few 0 cards, not to mention Megalixirs and Cures.
If you have the Roxas Enemy Card you can get some good damage in with it, but the preferred method here is to use Sonic Blade. This is regarded as the toughest battle in the game. He drop Riku Replica Enemy Card .
After you defeat Riku you’ll learn the truth about him, if the Riku Replica Enemy Card didn’t give it away. You’ll want to head back to Destiny Islands before talking to Riku since you’ll be in another boss fight. It’d be a shame to lose to Larxene after beating the Riku Replica.
BOSS - Larxene
Larxene comes packed with the Mega Volt and Teleport Rush sleights from the first time you fought her. Mega Volt is not that bad, but Teleport Rush still has a chance to beat you. Larxene also comes with a new Blade Storm sleight. To avoid this you need to be crafty or card break it. If you don’t avoid it, you get locked in the lightning bolt and your HP drains without you able to do anything. You also end up stunned which means you’ll get hit with her lightning attacks and likely die.
If you don’t want to use a Sonic Blade deck to beat Larxene then you’ll want Vexen and Jafar-Genie cards. Including a combo for Holy is great, it also never hurts to have a bunch of Cures. Larxene takes normal damage to sleights though, so there really isn’t a point to skip using Sonic Blade over and over. Larxene drops Larxene Enemy Card .
You acquire the Oblivion Keyblade Card after this battle and get access to the 13th Floor.