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.)
For Ark Mobile easy tranq arrows no need for infrastructure.
Tame a ptera and go searching the coast. Either use dilophosaurus bow or terror bird sword with better duds boots to kill and harvest the biotoxin. Then when you have a satisfactory amount of biotoxin go to one of the glacier islands and farm the penguins for polymer. Then comine 5 biotoxin, 2 polymer and an arrow for toxicant arrows. They deal much more torpor and the torpor of the creature rises for 10 second meaning if you tame runs away they most likely will knock out while running! I got 300 arrows in 2-3 hours.