From my experience, the difficulty you have with Marluxia partially depends on how you leveled Sora up.
In my opinion, the best way to go about it is to have a fairly high CP deck with a lot of higher-end quick-swing and good recovery Attack Cards along with a handful of 0 cards ready to break Marluxia's sleights and some Cure cards ready to make up for your lack of health and a few Clouds for a good measure of Omnislash fun.
I actually can beat Marluxia with my current deck (see above description) without using Magic Cards save for Cure and with my only Summon being Cloud with his Omnislash. But then again, Sora's HP is less than 200, so I definitely sacrificed major HP for my CP.
The reason I suggest having a higher CP deck is because of the need to break Sleights in Boss battles and the resulting need to be able to seriously punish the Boss upon card break.
To break a Sleight, especially with Marluxia and some of the bosses before, a 0 card is the best option as it is both the CP-cheapest way compared to performing Sleights and most guaranteed way to card break. ... But they aren't THAT cheap in CP points compared to other single cards.
And then, once you card break, the enemy is stunned and left vulnerable and the best thing to do when that happens is PUNISH THEM and I mean attack, baby, attack! ... And the best way to do this is to go at 'em like there's no tomorrow and to do THAT, you need some fast swinging action and a good recovery time never hurts. ... But those cards aren't cheap on the CP either.
Heck, NO good card is ever cheap (though Premium cards can be lifesavers if used right).
So you see, to be able to really battle against later Bosses effectively (again, this is just my opinion) without always having to reload, reload, reload (which leaves you nice and vulnerable to THEIR punishing), you need a fairly high CP with a fairly decent deck. Kingdom Keys can be great and all - heck, I still have them in my deck, but when you're battling a Boss, every second counts and their swing speeds are nothing to sneeze at compared to a lot of the other cards.
If you don't have a high CP/Attack-happy deck already at hand, I suggest creating a second 'Marluxia-specific' deck from what cards and CP points you have and experiment with possibilities. A suggestion is: Add as many high level, quick swing, and decent recovery Attack Cards as you can for attacking along with at least 3 0/zero cards for breaking Marluxia's infernal combos/Sleights (if you put them in the front of your deck, you can scroll past them when battles begin or when you do a card reload and easily scroll back to them when you need to break a Sleight) and pack along some Cure.
If after a lot of experimenting, you still can't beat Marluxia, you might simply need to level up some more and build more CP so you can get a more decent deck.
Or, if you have good enough CP and you're lacking in the 'decent cards' department, it might be time to bust out the Moogle Map Cards and try your luck at shopping. Sora only knows how many of my best cards came from cards that I bought from Moogle shops.
But in all honesty, unless you didn't build up your CP AT ALL, you should be able to beat Marluxia. Might take some experimenting with your deck and a bit of luck, but it should be possible. I mean, building HP over CP IS an option in the game, right?
Let me know how it goes and if you're continuing to have trouble, feel free to leave me a message (tell me what kind of cards you have and what your CP number is and your HP level is) and I'll see if I can offer any other advice?
Right now is rather convenient as I am on my way to beating Marluxia again as I am replaying CoM myself.
