An explanation of the strategy for the Archangel Sandolphin boss fight.
At the beginning of the battle, Sandolphin creates deep water near itself that you will drown in, so keep your distance at first. Then it will shoot some stars out. You can avoid these by diving underwater, or just swimming out of the way.
When a platform appears, stand on it and hit Sandolphin with your sword. Repeat this until the platform disappears.
When a moving platform starts going across the screen, swim near Sandolphin so that the platform will go under you, and attack when you are on the platform. Sandolphin will shoot a large ring of stars after this, so dive underwater to avoid it.
If Sandolphin is the boss of the fourth dungeon or later, there will be a second phase of the battle. In the second phase, Sandolphin will shoot many stars out of its head and tail while spinning around the room. Briefly dive underwater to avoid the stars. Avoid staying underwater too long, or you will lose health. When a platform appears, stand on it and hit Sandolphin's head and/or tail segments. Repeat this until the fight ends.