Question

I want my Pokémon to get a lot stronger for when I play Black 2 so I want to find out how to get. Is it automatic when you begin the game or you have to do something?

Answer 1

No it is not automatic,Yes you do have to do something to get Challenge Mode.

Challenge Mode

Unlocked: Defeat Champion - Pokémon Black 2

Source

Team AB.
&
Team Swagg.

Comments

But at the beginning of the game Hugh can be battle during this mode so does that mean after I defeat the champion I start the entire game over again.

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Someone else is able to turn on Challenge Mode for you if they have it.

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No, you unlock a "key" which unlocks Challenge Mode when you beat the champion. Then, you can use the Unova Link in the menu to connect with other BW2 games and transfer it.