Complex sentence: Li Shang was supposively execute Mulan by law but decided to spare her life as a debt for saving his own leaving her on the mountain while they make their way to the Imperial City to report the news of the Huns demise.
Split into simpler sentences: Li Shang was supposed to execute Mulan by law but decided to spare her life as a debt for saving his own. They left her on the mountain while they made their way to the Imperial City to report the news of the Huns demise.