There's two ways to win this, protect Liu Bei up until he retreats or just focus on defeating enemy officers and smash Cao Cao when he appears. If you aren't Zhao Yun or Zhang Fei it's pretty easy to trigger some of the neutral bases to join your side but since we're Zhao Yun it's not happening here. He get special dialogue and morale increase if Zhao Yun approaches Zhang Fei while he's guarding the bridge and reunites with Liu Bei.