Question

I battled kyurem and accidentally killed it because i didnt wanna waste my master ball and i didnt save.

Comments

You don't 'kill' pokemon
You knock them out

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It's not a waste of a master ball,
You can get another one.

Answer 1

http://pokemondb.net/pokebase/49478/if-you-ko-kyurem-can-you-have-him-come-back
http://pokemondb.net/pokebase/59063/how-do-i-rebattle-kyruem-in-pokemon-white
http://pokemondb.net/pokebase/52874/what-happens-when-i-defeat-kyurem-in-white

Comments

I already rebattle the elite four

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What you do if you defeat Kyurem is then battle the Elite 4. Once you beat them, go back and Kyurem will be there.
If this doesn't work then something is wrong with your game.

Answer 2

well sense you said you didnt save
have you tryed going back to where kyrem was before
it might be still there