This is a amazing thing! Like really, For example when you tame a Wolf, you can feed it biotoxin and it Will increase the torpur up to 80. Thats how i tame most animals, though its easier when you're in a tribe, they might have baryonix's or even a basilisc! It'll be much easier when you have a tribe and many biotoxin, trust me.
Either use a bow and arrow to shoot the jellyfish from the beach, then swim in and kill them, or swim in and then out to lure them close to the shore, where you can use a pike to kill them.
Basilosaurus don’t get electrocuted by jellyfish, so if you go swimming around on a basilosaurus and attack jellyfish, you can get a decent amount of bio toxin.
(I haven’t noticed that certain weapons or creatures get any more bio toxin than others.)