Tips for the bosses:
Boss 1: Use the power upgrade. Whenever you get a chance, swipe him with a full swing of power, then back off.
Boss 2: These are very annoying, and since all they do is fly all over the place, use finesse. Once your health gets low, using your finesse to go back and forth across the screen should allow you to recover health- then attack them once your health is full.
Boss 3: This guy is extremely slow, and so power will again be the best choice. His attacks are fairly easy to dodge, be patient!
I suggest you start with the easy setting, max out each attack, beat the game, then try to beat it on insane mode.