Play the Battle Of Wu Zhang Plains on the hard difficulty setting. It is better to do this in Musou Mode than in Free Mode. Wu Zhang Plains is a five-star battle, but in Cao Pi's Musou Mode it is only his fourth stage. Thus, if you do this in Musou Mode the difficulty will be lowered somewhat. First, defeat any three generals (not sub-generals) quickly. The ones that all grouped in the center make good fodder, as except for Wei Yan they are all generic. Once you kill three generals, Zhuge Liang will pass away. Continue fighting normally. At this point it would be a good idea to seal the enemy's supply route over to the east. Soon you will see an event where Jiang Wei lays a group of automatic crossbows on the central platform. Now all you have to do is get up there and break all of them. There will also be juggernauts floating around, not to mention archer towers -- be careful. Once you have destroyed all single automatic crossbows, the precious item box will conveniently appear in the middle of that central platform. Pick it up and finish the stage to earn your fourth weapon.