How do you beat the river styx on mythology island in poptropica?

The "sea monster" is the Hydra. It has five heads that try to strike you. When it tries, jump up before it hits you, and land on its head. You have to do that to all five of the heads to defeat it. Then you can get its scale.

(TIP: Don't take long! The heads will start to wake up again if you take too long.)