Aron. All Petalburg gym uses are normal pokemon with normal attacks., Aron's defense should be around 100 at lv.29, so Norman(Petalburg's gym leader) will only do like 5 damage a turn. Just keep using take down and you'll get there eventually.
my bad, i thought you said Rustboro, lol. yeah what they said, use fighting, but i also used Kadabra against Norman, SP Defense are really not that great on his pokemon
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