Select the battle of Tian Shui (playing on the Shu side). Once the battle begins, capture Nan An and An Ding Castle. To do this, simply eliminate Xiahou Mao and Cui Liang. Now, to acquire the Zhuge Liange, chase after Jiange Wei as he retreates from Tian Shui Castle. Defeat him when you catch up with him, and an item box will appear holding the weapon you seek - Zhuge Liange.