The leader in Hearthrome is Fantina. She uses ghost-types, so the easiest thing to do is use a dark type since they're unaffected by ghost type moves . Try shinx, or his evolutions. He worked well for me. Hope this helps.
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go to hearthome city
I purte sure you have to beat the gym leader in Veilstone city
You will have to beat the Hearthome city gym leader to use defog outside of battle
After you beat fantina at the gym in hearthome city she will give you the TM shadow claw.
You have to beat the Hearthome gym leader then you can get through. (do NOT use normal type moves on her because she has ghost type poke'mon and normal type moves won't effect ghost type pokemon).