Despite of what you could think, this turns out to be a semi-easy battle. If you follow the tips I have below, I bet you will have a better chance defeating Marluxia in the first battle against him with just four easy steps to follow!

1. Put a lot of keyblade cards with the values of 7, 8 and 9
2. Put Cloud cards with high values (7, 8 or 9)
3. I'd recommend at least 4 healing cards (with higher values)
4. Finally, put 4 or 5 of the cheapest cards with value 0 that you can find.

Now, attack him with the keyblade cards of high value with combination's of 3 Cloud cards. It is important not to let him break the cards because he has combo cards of high value, or he sometimes uses cards with value of 0. To counter this, use your cards with value of 0 to stop his combo attacks. Be sure to heal yourself before it is too late! It is important to be extremely careful with one of his normal attacks, in which he raises his scythe and quickly makes an attack with it -- this seems to cause you the most damage of all of the attacks he uses. Apart from that, just keep on attacking him with everything you've got, while remembering to heal yourself.

Another strategy would be to use the following cards:

Kingdom Key (9) x11
Kingdom Key (8) x6
Kingdom Key (7) x4
Kingdom Key (0) x4
Cloud (8) x2
Cloud (6)
Cure (9)
Cure (7) x2
Megalixer (7)
Dragon Maleficent
Neoshadow x2

As soon as the battle starts, activate Maleficient's enemy card and begin attacking Marluxia like crazy. If he uses his Deathscythe or Petal Dance, use a zero card. After your Maleficient card is spent you can activate one of the Neoshadows, but do this only after reloading. If you find your health running low, play Riku and use Curaga. After you play Riku, feel free to do an Omnislash if you like. Soon enough, Marluxia will die.

b a c k   .   c l e a r   .   f o r w a r d