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you fight ghetsis
he goes with ghetsis
beat n then you have to battle him
In the dreamyard after you beat N and Ghetsis.
he is at the pokemon league after you beat all four elites and N
you can catch it in giant chasm after you beat N and ghetsis.
You have to beat the elite four. Then beat Ghetsis and N.
After you beat the elite four, there's a zoarak out side victory road . Follow that zoarak and he will lead you to n , n gives you the black stone in pokemon black , and the white stone in pokemon white. Get the stone to dragonspiral tower and it will revive. Catch the legendary pokemon and then find kyurem in giant chasm. Before you beat the elite four, you will have to beat white/black kyurem after you beat it, there's the gene wedge on the floor.use that gene wedge to fuse reshiram/zekrom to kyurem to get black/white kyurem.
Have fun do some battle subway or do wifi battles
If you mean battle him, you do that after you beat N in N's Castle.
In Pokémon White 2, during the time in which Ghetsis is using White Kyurem, yes, it will remain as a White Kyurem if you don't defeat it but it must be defeated in order for you to continue. Once you beat it, White Kyurem will separate back to Reshiram and Kyurem but they will be available after beating the Champion.
You just have to train your pokemon. I did and defeated him with no problem. Mainly with Reshiram and Focus Blast.