I'd recommend you go with a giant squid to tame them because you can grab them also the basaileosaurus is not the guy to tame this guy in genesis becuase x_mosasaurs they are at a low depth the basaileosaurus can go only a specific depth till it starts to take damage. Also if you dont use a squid make a trap a 2Γ—2 beheamothgate with some gateways in them only put a gateway in the back when the mosasaur is in there

More Mosasaurus Taming & KO Tips