Unless you have projectiles to shoot at jellyfish from afar, only kill them with tuso, basilo, or desert titan (very unlikely you'll have one but they can function normally underwater. They can't be transferred so you'll only be able to kill jellies with them on maps they are summoned via admin command). These are the only tamable water dinos that resist the sting. DO NOT USE MOSA OR GIGA, THEY ARE AFFECTED. you may be able to kill one or two with other dinos, like mosa, plesio, sarco, or spino, but don't risk it when fighting a group of them. Another alternative is to use an iguanodon and harvest the poisonous red mushrooms found on both abberation and the abberant cave on valguero (they are more common on abberation) . Make sure you bring a gas mask to be safe. I hope any of this info cleared some things not already stated.