I used my basilosaurus. It has 21k health so I just agro'd the manga to my basilo and crouched on my big bois back and fed the manta as it swam around attacking my basilo. The wait time between first and second feed was the longest as with all passive tames. Modded server with 10x taming.
Tame a kaprosuchos and swim to one of these bad boys and do it's alternate attack it will grab it in it's mouth. walk onto the shore where its safe and have another person feed it while in the gators mouth (if you unmount it drops it) it wont attack or die because it's technically not on the shore.