The thing with sleights is that if you build your deck right, you can have another sleight waiting for you once the first card used is dropped out from subsequent use. Meaning, you can't just spam them one after another without thinking a little further because you'll just run out of cards.
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I know of Riku's bonus story, but I wonder if it should be played in order to understand his role in II a little better? I know that some Organization members are bosses there who aren't in Sora's story.