To tame a Shastasaurus, lure it to the ocean surface using 1-3 tamed Ichthyosaurus as bait. Once surfaced, dive underneath and use a Sickle to scrape three leeches off its belly. After clearing the leeches, feed it Extraordinary Kibble (or Raw Prime Fish Meat) at its mouth. Repeat the leech-removal and feeding cycle until taming completes. More Ichthyosaurus nearby extends the time it stays surfaced.