it depends on which sinnoh game you are playing. if you are playing diamond or pearl, then you have to beat maylene (veilstone) and wake (pastoria) before you can return and challenge her. if you are playing platinum, then she is the third gym leader rather than the fifth.
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Eterna's gym leader can be found in the Eterna Gym After defeating the three trainers hiding in the trees inside the gym you can find the gym leader at the northern (upper) part of the gym.
go into oreburgh mine and talk to gym leader
Go to Blackthorn city. The gym is there
You find him in the last Gym in Sootoolis City.He is the last Gym leader
The Mahogany City gym leader is not in Cianwood. He is in the gym in Mahogany, not anywhere else.