Go forward at the beginning of the battle and capture the southeastern garrison by defeating Guan Yinping. Your next task is to defeat both Mi Fang and Fu Shiren, but they will likely retreat before you can get to them. After that, start heading north and east, towards Guan Yu.
Cross the central area of the battlefield, only to have Zhou Cang start using catapults to pelt the area with rocks. Defeating him will make them stop, so hunt him down. Head back to the central area, as Guan Ping and Guan Suo start heading towards the floodgates. Guan Suo will likely shut the gate, so he'll need taken out first.
If you don't make it in time, Fan Castle will be flooded and it's likely that a lot of officers will make their way to Cao Ren. Should that happen, you'll need to go to him and clear the area of officers. Otherwise, head towards Guan Yu's main camp, which is likely accessed from the southern gate. Guan Xing controls some arbalests in the area, so defeat him to stop them. Defeating Guan Yu will put an end to the battle.
You need to win the battle and make sure that Lu Meng doesn't get injured to get this star. This is easily done by playing as Lu Meng, as you can control the situation and not have to rely on the AI. At the beginning of the stage, make sure you defeat Fu Shiren and Mi Fang to make them defect to Wu. Whatever you do, you cannot enter Guan Yu's camp in the southwest from the east, as this will trigger the arbalests and Lu Meng will get injured.
Fu Shiren will say he knows how to enter the camp from the north, which is where you need to enter. You likely have to stop the flood attack by defeating Guan Ping and Guan Suo, which requires you to be quick, as once Guan Ping gets beyond the gate there, he will likely trigger the flood attack.