I see your problem. I can fix that.
You see, Kyurem is not directly if front of you when you enter the second cave entrance. You will aproach a maze full of tall grass and trees. Sounds simple, huh? But its not. If you go to the middle of the maze, a little to the left, go right in the trees. Navigate your way and find a way to a small area with a small puddle. Once you approach here Kyurem will yell as it realizes your here. It will fill the whole maze up with snow, crating a white area with no obstucles. If you go to the top right you will find stairs and another cave entrance. Enter and find Kyurem.
Remember to save - just in case. Dusk Balls are very effective since this battle is in a cave, so bring at least 15 along. Press A and fight Kyurem! First, lower it's HP with move that are weak against in to avoid accedently KOing it. False Swipe is very helpful to have when Kyurem's HP is red. Use a ball once you have lowered it HP as much as you can, and if you are able to give Kyurem a status problem, do so.
Hope this helps, and enjoy your Kyurem!