Don't lead your new tamed eel home while skimming the surface on your "whale"... The eel will surface itself at some point and instamtly die from being out of the water. Took me 3 eels this way before I figured out how they were dying without getting into combat.
Masstame 150's til you got 25 of them, use them together with a swarm of 50 mantas backed up by mateboosted basils, mosas and tusos.
Absolutely insane, ofc have them covered by some tek and heavies set on players only.
Have them all following a passive tame on low distance (could be an angler), and have them set on attack-my-target, and aggro range high.
Have a trapped tame as scout on aggressive and aggro range high, when it aggro towards any dinos or players, it will send the whole force to kill and stun the target, when target is out of range or dead, they will come back to base.