Just so everyone knows, Marluxia is a GUY. Stop the use of referring to him as a her.
Anyway, Marluxia is easy, and the thing is that the highest card he'll ever use is a 8, or at least that was in my battle. He doesn't even stock cards, and most of his cards aren't strong at all. Just break his card, and when his defense is down, unleash a can of wh00p @$$ on him. It's really a battle of patience because of all that life, but keep in mind that Marluxia IS weak to physical attacks anyway, so that should make the battle a little easier. Even the Kingdom Key should do a noticeable amount of damage. What you want to do is break his two scythes/arms in the beginning, and just keep dodging and attacking. When the time arrives, jump on his back and just wail on him. If nothing else, pack the Vexen and Oogie Boogie Enemy cards and use them when you're low on health, but in truth extra healing shouldn't be necessary. Heck, if you can get past Riku 4, you're basically beaten the game in terms of difficulty.