You have to get their Pokégear number, check the related links below.
Battle the rest of them
yep
beat elite four rematch
It should be on Route 25. If Suicune faints, or you run away you can have a rematch at the Burned Tower.
you cant! but if you want a rematch just go through the elite four again
On eather game, sadly, you can't. But on Diamond, Pearl, Platinum, Heartgold and Soulsilver you can...
In order to rematch Jasmine in SoulSilver, you will need to get her Pokégear number which you can accomplish by going into the Diner in Olivine City between the in-game time of 1PM and 2PM any day of the week, Jasmine will be there so just talk to her and her number will be registered. After that, just call her on the Pokégear on Wednesday during the afternoon and she'll head off to the Dojo in Saffron City which is where you will be able to rematch her at.
i suggest, just raise your Pokemon
No one leads it. Gym Leaders can be battled here once they have been registered to the Pokégear's phone and have been called for a rematch.
you have to go to the victory road gate between 16:00 to 18:00 any day and she will be standing there. then you can rematch her on Monday mornings
dark
there are many things to do. you can go to cerulean cave to get mewtwo, or you can go chase after latios. there are many other things to do to like rematch gym leaders to though.