How do you defeat the monster in the underwater cave on mythology island?

The "gnarly monster" is the five-headed Hydra. When you approach it, it will strike out at you with one of its heads. Before it hits you, jump high into the air. When you come down, land on top of the head, knocking it out. Repeat this for all five heads, before any can wake up. Swim beneath the Hydra to get a scale (one of the 5 sacred items).