Fighting Larxene and Axel was difficult until you figured out how to dodge the attacks. Once you figured that out, they were easy.
Marluxia wasn't difficult at all. I had much more difficulty with Vexen, and, to a slightly lesser extent, with Seeker of Darkness Ansem. The former, because he could block, and his ice spear was terrible. I hated fighting him in Twilight Town, though he was much harder the first time around. The latter because if you got caught in some of his attacks, you'd be killed instantly, and because of his constant card breaking.