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!
Up to 15 tentacles grow from each corner of the bell and can reach 10 feet in length. Each tentacle has about 5,000 stinging cells, which are triggered not by touch but by the presence of a chemical on the outer layer of its prey.
If you wanna kill these, but you're a beginner, just use a slingshot. Or if you can, use a bow and stone arrows. Harvest their bodies, they can't shock you if they're dead. They give bio toxin, which is a very effective narcotic.