Probably a good idea to apply the same rule for fighting Onycs, that is, avoid melee. Their acid can hit you on smaller mounts you would usually bring in caves. They can be hard to spot in water and against uneven cave walls, but their oil-puddle-like iridescence can give them away if you look hard enough.
DO NOT ENGAGE OR YOU WILL DIE! I was hunting araneos when this thing came out of nowhere and three shot me! All my dimorphs were literally devoured when I came back. It is like centipedes in real life but instead of killing you slowly with poison it kills you with bites quickly. Do not equip anything lower than Chitin or you will get one shot. Make the up arrow green so people can know 🙏🏻