Start the battle by going forward and taking out any officers that are in range. Eventually, the carriage will turn around and you'll need to go and catch up to it, which will cause Guo Si to appear, so defeat him to make the carriage go the right way again. Continue taking out any other officers until you get to the right side of the fork.
You should also be getting messages about Yuan Shu's men appearin at the byroad, which is the southern checkpoint. Taking out the officer and base captain there will put a stop to more men appearing. Go to the locked gate and take out Yuan Shu to make it open, which will make the carriage carry on to its next stop.
When it does stop, it's because of boulders that start falling. About this time, Lu Bu will appear at a checkpoint behind the carriage and it's best to leave him alone, as he can defeat you rather quickly. Dodge the boulders and continue to the designated spot, defeat all of the officers to eventually put a stop to the boulders.
Continue to the next area, where there's a fog, which will limit your in-battle map to the closer one. Also, the Emperor has been kidnapped from the carriage and you need to find him. If you do find him, try and clear the area of officers before you meet up with him, as he will follow you around afterwards. Bring him to the escape point, where Zhang He and Yuan Shao will appear. Defeat them both to make the Emperor escape and finish the battle.