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.
Surprisingly in mobile ark, if you attack achatinas with raptors, as I have learned from my own experience, you can get angler gel. I have never gone past the shallows in the ocean, yet I have multiple stacks of angler jell. Hope y’all can find this helpful. Don’t know if it works on other versions of ark though.
Get a basilosaurus and get the manta you want to tame, agro it to the balsilo and you will be able to tame the manta since its attacking the basilo instead of you. (The basilos food will drain quickly so get it some food)
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.