I've only beat Marluxia and Lexaeus Data at somewhere (I think) between lvl. 85 and 90.For Lexaeus all you have to do is use reflectaga and guard. Use reflectaga to block when he shoots the rock up from the ground, and when he does the upper slash, use guard then the reaction command, then beat the crap out of him. And when he starts running towards you, use reflectaga, or he will grab you and if he does he will do a lot of damage with the combo he will do on you.
And for Marluxia, try getting Glide Lvl2, and using Guard, and have a high lvl. Use Guard to defend against his slash attacks, and after guarding against his one attack when he draws you in and starts spinning and slashing you, use the reaction command. Glide will work for dodging the three circles of Darkness he makes, the spell where he makes the Darkness spin around you and it explodes, and when he makes Darkness rise from the ground. Reflectaga can also help with this fight as well.