1.tame a dolphin (ichthyosaurus)and ride it

2.go to the shasta and it will do some communication with the dolphin

3.then it will follow the dolphin so make it follow you to the surface of the sea

4.there will be some things attached to the shasta's body and you have to remove them using a sickle

5.after you remove them go to it's mouth and feed it the kibble

6.repeat until tamed