To defeat all of the bosses, you will need to clear all optional boss, and then you will need to do two runs, through the New Game + 0. The reason why is because you need to do the Radiance ending, where you can defeat Damarose and Adyr, as both of those bosses require you to cleanse the beacons. In your next play through, you will need to Elianna as part of the Umbral Ending, which requires you to use items on Damarose to progress the storyline. From there, it will allow you access to fight Elianna, which is the final boss to get the Umbral Ending.