Which boss fight are you doing? Do you have any of those helpful potion things you can use? Not actual potions - but there are two that will cast haste/shell/veil/etc.....before the start of the battle. Sometimes that is helpful to do to give you the upper edge in the beginning of the battle - and often times that can make a world of difference.
If you are still stuck - let me know which boss it is (I can't remember going just by the Chapters) and I'll tell you how I did it. Which may not be the best or easiest way - but it worked for me.
I finally beat the chapter 9 boss after over an hour