Play Zhang He's Legend. As soon as the level starts, follow Wei Yan until you see a way down the mountain (the one where if you go down, there will be Guan Ping's ambush). As soon as you see the way, hit Wei Yan with your C4 to the way down. Then, go fetch him. As soon as you do, Guan Ping will activate the rock slide. After that happens, Wei Yan will remain still, not moving. Finish him off. After you defeat him, Zhang He will say something, and the message ''Zhang He has gained the final weapon Infernal Talon!'' will appear.