Merman Fly is the upper half of a pale, armored humanoid with wings.
Merman Fly has a basic pattern. From the right, it'll fly above the Master and launch energy bolts into the water. Touching either the bolts or the splash they cause will inflict damage. Merman Fly will keep throwing bolts as long as the Master is to the left. When the Master is to the right of Merman Fly, it'll fly up to the left, back down, and to the upper right to repeat the pattern.
It's possible to damage the creature as it's throwing energy with jump-attacks. It also leaves itself open to attacks during the pause when it dives down on the left.
Magic isn't very effective. Merman Fly is too small and mobile for Magical Stardust to rain damage upon, and it stays too far away for Magical Aura.