I got attacked by ten of these jellys and i've read that once you or your dino get attacked its over, well it's not. Simply stab at the jelly(i use pikes) to get them off dino.and if you yourself are the victim just take some stimulant to get knocked out
P.s always have stimulant, land or sea. Its a life saver
To kill it, use 2 tranq arrows(I only use tranqs for underwater tames.). It will die quickly. There is a rule saying I'm not allowed to ask for an update. LOOPHOLE. Submit a tip saying my knowledge. Tell my story, I am AnonymousUserYT.
Don't attack it with any sort of weapon, even stone. It will electrocute you. If you need its bio toxin, just attack it with fists, and make sure to stay far enough away so that it doesn't sting you. I suggest finding a group of these guys near the surface so that you don't have to worry about bringing any tames, or looking out for aggressive dinos. Hope this helps :>