It's weird, I still haven't been able to beat "Mysterious Figure" or Vanitas Sentiment, but I've lasted significantly longer on MF.
Vanitas kills me in one hit every time and MF seemingly "takes his time" killing you, with each lightning-fast attack of his taking chunks outta your health bar. Then I heal myself, dodge roll around a little bit, and he catches me while doing the same attacks. Both are impossible for me at this point, and I don't have the will or patience to try again. Must have tried at least 70 times fighting MF, barely getting him down to half-life.
Come to think of it, I never finished that. Sorta lost interested during Terra's story when I realized how moronic he is.
Does it get better later?
Definitely. Like I said in the KH thread, the ending of Terra's story IMO, is the best part about the game. It was really well done.