For Marluxia, just attack his claws with regular attacks, don't use sleights, that'll just take cards away.
Then dodoge his air attacks, and when he charges at you,k break it, so you can jump on his robot.l Then just attack him. That's all really, took me a while too.
For Ansem, I wouldn't listen to Splinter's advice, sleights make you lose cards. And besides, Riku is stronger when using regular attacks, especially in Dark mode. To break, just use the zero cards. If you upgraded his attack a lot, then you should hurt him a lot with regullar attacks, not sleights.