As a Selkie, when revisiting Mount Vaal in Story mode (the second boss battle of Mount Vaal), you will encounter Al instead of the giant bird. Back up into the south corner as far as possible from Al and turn around. Al cannot attack you from there. Repeatedly jump and shoot to take him out without getting hurt. If timed correctly, you can hit Al at the top of a single jump, which is faster than double jumping every shot.