After you beat your game, you're told of a random Zoroark that's going around Victory Road. If you follow after it, you're going to come across the remains of N's old castle from Black and White, the previous games. There, he'll challenge your strength the first time with only his Dragon types, Reshiram and Zekrom, both level 70.
You can also take him on for a second time - if you visit N at the Ferris Wheel on Fridays, he'll rematch you at the castle again. His team is different - and it changes with seasons.
Spring: Politoed ( 77 ), Lanturn, Tentacruel, Omastar, Kabutops, Starmie ( 75 )
Summer: Ninetales ( 77 ), Arcanine, Rapidash, Shiftry, Ninjask, Leafeon ( 75 )
Fall / Autumn: Hippowdon ( 77 ), Gastrodon, Cradily, Armaldo, Scizor, Rhyperior ( 75 )
Winter: Abomasnow ( 77 ), Vanilluxe, Cloyster, Mamoswine, Froslass, Glaceon ( 75 )
At 'N's Castle after beating the Elite Four.
You can go there after beating the elite four.
When youre versing the champion
in the giant chasm after beating the elite 4
No, you cannot.
route 7 after beating the elite 4
You can battle the elite four at the pokemon leauge.
You will receive it after beating the elite four and defeating Team Plasma.
Yes in Pokemon White after you defeat the last member of the elite four you are forced to battle Zekrom. You battle Reshiram if you have Pokemon Black.
Yes, but first, in white, you must defeat the elite 4.
you can transfer Pokemon from Pokemon platinum to black and white but you can't trade from black to platinum
Yes, but you need to get the national dex first by beating the elite 4 once