I think you did rather well on this piece. Really seems to capture the essence of the boss battle without all the harsh and extra sounds from the original. It still gives you a feeling as if you may not get Kyurem as well since it is a legendary Pokemon too.
The piano seems to act on multiple levels. It goes fast which gives it a dramatic feel as if you had a limited time to try catching the Pokemon too. Then the lower notes play a great piece of the song that gives you hope that you may just be able to catch it as well. Pokemon are just a lot harder to get without a Master Ball too.
Overall, beautiful piano cover.