This is Pokemaniac speaking. Here's an idea to stall for Steelix. Take that weak Base Onix, some Fighting Energy, and some Metal Energy. Onix's Harden states that all damage 30 and below is prevented, but other effects still happen. Damage effects come first(here, Metal energy), then the Harden effect. Add Defenders, Item Finders, Potions, Berries, Gold Berries, and anything esle that heals could keep Onix going for a loooooooooong time.

 

The Math: Onix with 4 Metal and 2 Fighting uses Harden. First of all, -40 for the Metal, then after that, 30 and below is prevented. That essentially means in this case all damage 70 or less is prevented! Of course, 4 Metal is hard to get, easy to lose, so back-up with Eco and Energy Charge, but still try to get Steelix ASAP!(It's still interesting to get a chance to see Onix immune to Scyther and R Zappy! Scyther could Swords Dance, though.)

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