Then you have practically completed the game. Have you done the Team Galactic bit in Stark Mountain? Maybe you should try to migrate over the regis and get regigigas. Or how about the 3 birds and cresselia?
Finishing the Battle Frontier in Platinum means obtaining all Silver and Gold prints handed out by defeating the Frontier Brains.
No, Pokemon Sapphire Version does not have a Battle Frontier. However, Pokemon Emerald and Pokemon Platinum do have the Battle Frontier.
no you can not get exp in the battle frontier in Pokemon plat
No, there is no battle frontier my brother (Pokemon master) says so. their is however one in platinum.
If you see a Pokemon in the battle frontier you have not yet seen, it will not record it. The only way you can add that Pokemon to your pokedex is if you see it outside of battle frontier.
It's in the battle frontier at the battle area
No, you cannot.
beat the frontier brains
No, the Battle Frontier can only be found in Emerald, Platinum and HeartGold/SoulSilver.
You unlock Silver Prints by beating the Frontier Brains in the Battle Frontier in Platinum.
There is no battle frontier in Pokemon Pearl. Sorry, you'll have to buy platinum for that one bud
As far as I know of - you don't ever battle her.