Just keep trying. A big part of these bosses, just like in Dark Souls and Monster Hunter, is recognizing the attacks and successfully avoiding them. You also need to be on the lookout for any openings that be can exploited, whether for attacking yourself or healing. The right charm setup can help you make the most of these openings. Patience is also very important.

For an example, last night it took me upward of an hour of attempts to defeat Sif in Dark Souls. I was constantly getting killed and then it finally clicked. I realized what attack was the biggest problem and I keyed into the tell that Sif was about to use it. I then kept my distance during that attack and the fight turned in my favor. The fight was then a game of being patient and exploiting the openings during other attacks. It was incredibly satisfying when I struck the killing blow.