Make a taming pen at the bottom of the ocean near a plesiosaur spawning place made of stone 7x6x6 with sealings,foundations,doorframes and a behemoth stone gate.Then use a tough underwater mount to kill the plesiosaurs you don't want, because they spawn in groups of 2 or 3.After that kite the last one into the pen,get out using a doorframe,close the behemoth door and you can hit it easily.Use this strategy to tame plesios and even harder dinos like mosasaur.

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