Well, this is what worked for me:
Try paralyzing or making Kyurem go to sleep. Then try to do some damage on it, but make sure you don't kill Kyurem. Once Kyurem's HP is Red try using a Dusk Ball because it works better in caves. If that doesn't work and the battle is taking forever try using a Timer Ball because the longer the battle the better the Timer Ball works.
Hope this helped =)