Here's a tip in killing a cnidaria without having to kill it with any weapons or dinos:

•Get in the water close to them(not so close unless you want to die)

•let them follow you

•get out of the water

•They'll be now almost out of the water

•Get back in side and let them turn

•They die because of their turning issues

That's use full to get them Biotoxin (Note that I don't know if this works on any other devices,only I know it works on mobile)