When he is in his machine form just use individual attack cards and strong magic like firaga. Also stock up on a few cures and an elixir or two. Use at least one three card Simba attack and multiple sleights of Cloud's omnislash. Marluxia's second form I found easy because all that you have to do is use the right cards and you have to be able to shuffle through them fast and pick the ones you need. You have to block all of his attacks and then attack him. One of his attacks he uses knocks all of your cards out of you but it is a good thing and a bad thing. It's bad because it cases damage but it gives you all of your cards back. After you defeat him you beat the game.
The easiest way that I know of to gain exp is to use the sleight tornado multiple times and try to pick up all of the enemies that you can, but if you are in a battle that has no fire type enemies use the move Mega Flare. I think you learn that at level 57 but if you have to work your way up to it it's totaly worth it because you will wipe out all enemies by using it one to two times.
Well I was on Lv.56 when I beat the first Marluxia (Which was an illusion) with a mash of many magic card combos. Like Firaga,Blizzaga,etc. Anyway after you beat the first Marluxia a really good deck to go up against the next two (you don't have time in between to modify your deck..) is one with the sleight/combo "Final Flame" (I believe thats the name for it) Anyway for this sleight first you've got to learn it by leveling up then it's the sleight that you use one "Stop" spell and two attack cards. This attack I consider it ""DEADLY"" for any boss,not only does it freezes the Boss for the time period you attack but you also take about 1 life bar off of your opponent (Well at least I did since I was lv.57). Well in the last battle it's pretty rough since Marluxia has several sleights so what I used for my deck was:
1- Stop,Attack Card,Attack card(4x)-(It could be good to have more than 4 of each of these)
2- Marluxia Card-(You get this after beating the first Marluxia and it lets you use all your sleights twice in a row for one whole reload, in other words if your lv.57 and use the Final Flame sleight with the Marluxia card already activated it equals 2 whole life bars by using only 1 sleight)
3- Item cards ( Elixir is good to have since it reloads all your cards, I'd say about 4 are good, 5 are better)
4- Attack cards (9x)- (Their value must be of 9 and you must have 9 of them,why? Well while fighting the last version of Marluxia he has a real ugly sleight called "Doom" and you have to counter his cards with your own if not...you die,so it's good to have attack cards of value=9 so you can counter all his cards,then after you can keep on hitting him with the Final Flame sleight)
5- Vexen (This card is an obligation to have,why? Cause if you die you have a second chance even though it's not with much HP)
6- Magic Spell "Cure" (6x) - (6 of these are good to have for all your HP healing needs plus if you use Vexen,you can heal yourself using Curaga)
7- Any Card you deem worthy (Any card you see that goes well with this deck is fine)
Ps: My max CP was at 875 so you can imagine how I fit all these cards here (even though they were at different specifications then the ones I mentioned earlier) so ya I hope this helps to all you who have problems with Marluxia.
Most people think that 9 cards are the best to attack with. No! A 0 is the best! The 0 takes more damage on the enemy, but be cautious! Use it after the enemy uses their card, or it's a card break. It helped me beat Vexen!
To get premium cards, you need to get alot of experience, I don't know how much, but when I was playing it didn't let me choose, but all the cards were spinning, press x and it stops and gives you a premium card.