The Mosasaurus is the fastest wild water creature, as well as the most powerful. Even with Scuba Flippers and 140% speed, you won't outrun it. There are a lot of ways to get one trapped, but if this is not a option or you are feeling adventurous, you need, aside from the previous attributes and items, Focal Chilli to increase speed and a good set of Flak Gauntlets/Leggings to provide defense if needed. With those, however, you can even chase it when time comes and it tries to flee!

Bring multiple Scuba Tanks since you'll definitely need.

More Mosasaurus Taming & KO Tips