To unlock you need to do 2 Targets on Battle of He Fei on Wei's side. This is pretty easy since no one is super tough. First take the camp on the island that you need for Target 1, take out a few generals nearby then head for the drawbridge in the west. You only have to beat on one side to bring it down, so beat it and then head for Lu Meng taking out more generals on your way. At some point he takes a back way east along a southern trail. About this time Gan Ning appears in the southeast corner. Ignore him, head for Sun Quan and take him and any other missed generals out. Taking out Sun Quan in 10 minutes gets you another Target and then just run around getting the stragglers before the clock his 70 minutes and you'll get all 3 and get Lu Meng.