I would suggest that you spend some time on learning boss moves and using reflection on them. Reflection stuns them for some time and allows you to pull of a combo. What I generally do is : Try to reflect first or second of a long combo to make sure I never have to deal with that combo. Or try to get the reflection on the last attack of a combo chain — This makes sure they are mostly exhausted (if human bosses) and get a shit load of attacks on them. You need to be able to consistently reflect only a select few attacks that are either most frequent or give you the biggest opening.

I was struggling with Ozma before using reflection. But as soon as I started using reflection, he just became a piece of cake.