In order to tame a Basilosaurus you need a croosbow and an acuatic mount that's enough strong to deal with mantas.

When you have all of this just shoot to a manta that's near the Basil and all of them will attack you, and here you wil pick the your crature and kill all of them.

Once you slayed all the mantas go with the Basil and start taming him pacificly.

Be careful because each time you feed it nearby cratures will come at you.

More Basilosaurus Taming & KO Tips