Just go around with your raft on orbit camera so you can see underwater and if you see some jellyfish close the surface hip in the water and kite them to the raft hop back on take out longneck shoot them harvest with pick got over 1000 toxin doing this
I use to go to the beach, swim out a little bit until it agro. Then swim to land again and the cnidarias are following me to land. When it I am standing on land it stops. Then I can easily kill it without taking any damage.