Tornadus is probably the best candidate for your Master Ball. Because it is a roaming Pokemon which can flee from battle, it's worth it for that assured capture. While it may not be as strong as Kyurem, Kyurem can nonetheless be captured the usual way with a bit of patience.
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Just be paitent and catch it with a pokeball!
MasterBall
Have a Fast Ball with you. This ball should catch at first time. If not, stack up on Ultras.
Any pokeball will work but an ultra ball is probably best.
A Master Ball