Best Way to get from XIAN to Zhangjiajie National Forest Park in the Wulingyuan?

I think you may try the direct bus for ZJJ from Xian Provincial Bus Station and the only one bus is at around 16:30. The bus takes about 16 hours to ZJJ.
if you do not want to take the tiring bus, you may try the trains. You need to take trains to Changsha first and the go to Changsha South Bus Station to take buses for ZJJ.