I have a tip to share on how to deal with the Mantas and Megalodons that are surrounding the area where the Basilosaurus roams. Make sure to go with a high level Baryonyx because of its underwater stun ability and damage in general. Make sure to damage one manta so they will all start following you. Then lead them away and kill them. Killing the Megs is easier in my opinion, plus you can gather them all in one place, stun them and start killing them!