I found it quite disappointing that the major boss battles were much easier than the lesser ones.
Like Riku and Xion... You'd think they'd be hard, but alas, they weren't.
Xion was alright in my opinion. Her first form was easy, her second form had a tricky and deadly ragnarok, third form had a deadly combo, and final form was packed with HP. What's more is the first three forms had the ability to fully restore their HP to max.
However, I was vastly disappointed with Riku and Saix's boss battles. Those were too quick and easy.