Hall Matron Argenta
no you do not
She is in the contest hall and when you talk to her you can go and battle her.
In Pokemon Diamond, you are not allowed to go to Leader Fantina's Gym. (Shadow Type Pokemon). So, get to Veilstone City and Battle Leader Maylene. She uses fighting type pokemon. It is the same with pearl. In Pokemon Platinum, when you just get in, a Buneary will bump in to you and a trainer will come. She thanks you and tells you to go to the contest hall. After your mom gives you a tuxedo/dress, there is a girl in a purple dress which is Leader Fantina. She goes back to the gym, and you are available to battle her.
In the concert hall to the left
you face six people then the seventh battle is the leader
You can battle the elite four at the pokemon leauge.
Cyrus the galactic leader
gym leader
you Battle Pokemon the talk to the trainer
Before the battle as he is a gym leader he has: Luvdisc Sealeo Seaking Whiscash Milotic
johto gym leader morty
she is on the contest hall in Hearthome City.