Use a basilosaurus to fight them. Since all their attacks involve electricity unlike the electric eels which can bite, they are doing no damage to the basilosaurus. Just spam the attack button and don’t forget to harvest their body for toxin too!
To farm these things to get the best keep-unconscious resource in the game, Bio-toxins, you will need a basilosaurus because they don’t get affected by jellyfish at all. You don’t need to ride it just target it to fight these things. After that stuff the toxin in a ceramic vase because you can preserve it in there forever as long as it doesn’t break.