It's not hard. It's just extremely broken and badly designed.Many bosses border on impossible without powering up your flurry attacks, as most of them can take as many turns on the trot as they like. But if you power up your flurry attacks, which are even more broken and badly thought out, you'll steamroller anything that gets in your way. Seriously. Even the last boss will go down in a handful of minutes at average levels. Essentially, every boss fight in the game boils down to a test of who can break the awful battle system more. And provided you go about things in the right way, you always win.
Yes, in case you can't tell, I'm not a fan of the game. Rarely have I played a game that squanders so much potential. Beautiful sprite art, lovely character designs, some great music, an all star voice cast (in Japanese) and an extremely cool central premise... all spoiled by an ultimately pretty crap story and some of the worst design decisions of any RPG I've ever played. Bleh.