There are 8 battles for this story which will free when you will finish some battle for the other stories (Wei, Shu, Wu, Jin).
The particularity of these missions, it's that you can't choose the character, he's imposed.
Zhang Jiao: Yellow Turban Rebellion, unlocked in Shu Story.
Diaochan: Battle of Hulao Gate, unlocked in Wei Story.
Dong Zhuo: Chase at Hulao Gate, unlocked in Shu Story.
Lu Bu: Battle of Xiapi, unlocked in Wei Story.
Yuan Shao: Disturbance at Guandu, unlocked in Shu Story.
Zhurong: Pacification of Nanzhong, unlocked in Shu Story.
Zuo Ci: Assault on Xuchang, unlocked in Wu Story.
Meng Huo: Capture of Chengdu, unlocked in Jin Story.
Some battles need to be unlocked on the hypothetical way.If you have unlocked ...
...you have all the stages in "Other Story"
They're pretty easy , except Yellow turban rebellion (3 kingdoms against you) if you play the game on hard or more in the first run.
You also unlock each character.
(Special thanks to Lamifa27)