Summarize:

During the beginning of the battle, Marluxia will cast Death on Sora. A counter will appear above Sora's head, and every time Sora gets hit by Marluxia's scythe the counter will decrease by 1. The spell is unavoidable, but if you avoid getting hit by Marluxia's scythe attacks, the counter will not decrease. If the counter hits 0, Sora will die.2. This is Marluxia's most basic attack; he'll slash at Sora a few times. Block (press ) this attack or use Reflega. Blocking this attack will cause you to lose the opportunity to increase Sora's death counter, but it will let you damage Marluxia. If you avoid this attack, you'll have a small window of opportunity to attack him with a reaction command that can either stun Marluxia or give Sora some numbers back if your death count is low. Marluxia will warp near you and attack you with the scythe, causing Sora to be pushed away or thrown into the air (aerial Warping Slash), then Marluxia will re-warp and attack Sora a few more times. Don't try to block this attack; just glide away until Marluxia uses a different attack. This attack is troublesome. Marluxia will warp to the center of the arena and plunge his scythe to the ground, causing three large holes to appear around the area. To avoid it, glide (while in the air, analog + ) around the arena and land on a safe spot. Marluxia will still attack you during this time, so use Reflega. This attack is strong enough to send Sora's HP to critical upon stepping on the affected area. Marluxia will summon rose thorns to encase your whole party, which deals a lot of damage. To avoid this, jump and glide around the arena until the spheres explode. Marluxia will spin on the ground and follow Sora around, causing damage to Sora and decreasing Sora's counter. Run around avoiding Marluxia as much as possible until he's done spinning. Marluxia will possess Sora and float above him; he will then summon small upgraded versions of his Death Hole, which will shoot up from the ground. Eventually he will continue with Warp Slash and a devastating finishing blow. When he starts floating behind Sora, quickly dodge roll around the arena until he starts moving again. Use Reflega to block his scythe attacks and keep casting the Reflega until he uses the finishing move If timed correctly, it will reflect back all the damage and leave Marluxia open for a combo attack.
Avoid his attacks after he casts Death. Block his Scythe Slash. Avoid his Warping Slash. Avoid the Death Hole. Avoid his Thorn Spheres. Avoid his Scythe Wheel. Counter Reaper's Possession.