Just pump health and when I mean pump health I mean PUMP THAT HEALTH! These guys die so quickly. I know it says on stats they have 320 health at level 1 but seriously they are so STUPID! in the wild, they sometimes jump in the middle of the most unuasual time and they instantly die even if the Manta is level 999 and even if its tamed it will only take MAXIMUM 9 seconds to die on land. AND ALSO IN BATTLES I DONT KNOW WHY BUT IT FEELS LIKE THEY HAVE HEALTH DEBUFF cause apparently they die so quick, especially to megalodons. Weird...
So just pump health.
They have terrible turning circle, approach one from above and feed it. Then let it swim circles around you. The occasional time it gets close you can predict its attacks, it “bucks” just before the attack, which is the tail swinging underneath. Quick swim up or back avoids it. Be prepared to get dizzy, long time between feeding
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.