Data Marluxia
This encounter always starts the same where Marluxia attacks with the scythe four times. Block all of these then run straight ahead towards the edge of the arena so you can hit him when he reappears.
Next, Marluxia will throw his scythe twice wherein you must block it both times. If you're having trouble deflecting the second throw, simply jump then dash as soon as you see Marluxia toss his weapon. He will be open for attack after this.
He summons several roses that can shoot lasers while doing scythe combos on the ground. You can quickly destroy the roses with Thundaga then start blocking the scythe attacks. You'll get an opening immediately after the combo ends.
Marluxia pulls you in and attacks with his scythe. He does this twice before leaping into the air to activate his armor. He becomes vulnerable to fire during his armor phase so hit him with Firaga to immediately remove the armor then quickly combo him. Going back to the beginning of this attack, the easier method to get an opening is to simply block his scythe when he pulls you in the first time. Use Counter Kick right after you block then start doing combos.
He summons several pillars from the ground then teleports attack you. To evade, you'll want to glide to one corner of the screen away from the pillars then wait. Block when he says "Ready?" the third time and hit him with Counter Kick. This is another opportunity for a full combo.
Once you damage him enough to say "The end is nigh", Marluxia will cast a 15-second Doom on Sora. At this point, you'll want to damage him until the situation command "Dispel" appears. Activate it t remove Doom. That being said, this phase is also the best time to do some really good damage on Marluxia as his behavior is very predictable. All you need to do is block and counter. And if you can attack him before he finishes saying "Settle Down", it is possible to keep your combos going until he only has 1 HP left.
When Marluxia has 1 HP left, he will cast Doom again but this time the counter starts at 99. He will the disappear to repeatedly throw his weapon. You cannot block anything during this phase so just dodge until reappears, then you can deal the finishing blow.
You will be rewarded with a Magic Weight for winning.