Make a small pin out of stone (it can't destroy stone) in water that is deep enough to allow the manta to swim. Lure the manta into the pin with a water animal or just swimming (whichever you prefer) and close the gate behind it. You can then feed the manta until it is tamed
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.