Well it depends what level and the moves it has learned, the elite four battle with level 60s and above so try and get higher than that level, if you have got a second Pokemon game get a friend or if you have two consoles use that to trade or transfur stronger Pokemon to
Your black 2
(Because you would have got all of your gym badges which alows you to use pokémon of any level) so it doesnt matter what pokémon you have its the level ( and moves ) that make it strong.
I hope my advice helped you out! :D