He doesn't evolve by leveling up. It's exactly like how you get a Kingdra. to get Kingdra you trade him or get him in a trade while he is holding a Dragon Scale. To get Steelix, trade him over and back or get him from a freind while he's holding a Metal Coat.
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Onix does not evolve by level. It can evolve by being traded while holding metal coat.
Level: Onix Evolves By Trading.
level it up
You have to trade him
onix has to be traded and be holding metal coat