How do you kill the monster that guards Poseidon's realm on Mythology Island?

You battle the five-headed Hydra in the "gnarly monster" cave within the underwater realm of Poseidon.

The Hydra will try to strike you by thrusting out one of its heads. When it does, you must be ready to jump before it does, then land on top of the head, knocking it out. Repeat this for all five heads, then swim below to get a scale from the defeated monster. If you take too long, the heads may revive!

(*outside and above the cave is the Giant Pearl, which you also need.) How_do_you_defeat_the_Hydra_on_Mythology_Island