Go forward from the beginning of the stage and open the first gate to the east, which prompts a Messenger to appear and start heading to the northeast corner. Defeat the officer in that garrison to open the next gate, then start heading towards the messenger. If you catch up to him before he passes through the next gate, then it should be easy to kill him and continue your plans in silence. Otherwise, things get hairy and more enemies will show up.
With the messenger hopefully defeated, head back to the green highlighted area and defeat the unit commander to lower the drawbridge. Start heading west after crossing the bridge and defeat the officer inside the garrison to continue your way. There will be ballistae in the next area, which is highly annoying while trying to take out the officer there. Your best bet is to keep running until you're underneath them, then continue to the right to get out of their range.
Zang Ba is the one controlling the ballistae, so go the next garrison only to have the gates be closed. An ambush unit with archers will appear, so take them out and then the gates will open once again. Go and take out Zang Ba to cease the ballista fire, then take out any other officers that might show up. If your unit has reached the floodgates, then Xiapi will be flooded. If he hasn't, then clear the area surrounding Xiapi until he does.
When the castle is flooded, a messenger will be sent out to go to the northwest corner and try to get some reinforcements. You should be in good position to intercept him, so do that to make sure none will arrive. All that's left now is to head inside Xiapi and go after Lu Bu, so go ahead and do that. If you defeat Diao Chan before Lu Bu, then he will become enraged and be tougher to defeat. The stage will end with his body on the floor.