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.
You may encounter this giant insect in cave, many people are scared just because it can one shot you even in full TEK suit, but don't get fooled by their looks and stats, they are slow and their attack recovery speed is lower than you, and they can shoot poisonous fluid from distance (super EZ to dodge) and their radar is short so best choice is...... You name it!!! A bow. Gauge them from distance and pow!!! Most of them are usually not that tough so good luck. (Don't fight them if other hostile are present around the area) don't mess up.