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After you get back from Cianwood, go to Jasmine in the lighthouse and give her the medicine for Ampharos. She will return to her gym after healing Ampharos, and you can challenge her there. She uses the Steel-type Pokémon so Fire and Ground are two recommended types to fight her with, since both types of moves are super effective against her team.

Leader
	Leader Jasmine: 3500 Magnemite (lvl. 30, 571 EXP) Magnemite (lvl. 30, 571 EXP) Steelix (lvl. 35, 1470 EXP)

After you defeat Jasmine, you'll receive the Mineral Badge and Iron Tail (TM 23).