I tamed one with a mastercraft crossbow and about 5-10 tranq arrows and around 90-100 bio toxin and some prime fish meat but it was only a level 15 or 16 use a bronto or a good berry harvester and just kill a Rex or something fro prime meat that spoils pretty fast I just killed an alpha raptor and then tame something like an archelon for bio toxin and then just tranq it out and give it extraordinary kibble, raw prime meat, or raw fish meat (regular fish meat and regular meat works too but it is not good and only does 8% taming) and then you have a mosasaurus.