Start the battle by fighting the soldiers in front of where you begin, as not too long after, two boats with two officers will land. Once those two are gone, Sima Yi will land a boat to the north, so head up there and defeat him. Next up will be a raid from the south, with Xiahou Dun leading the charge. There will be around 5 officers here and once they're finished, the prayer will be successful and the fire attack will play out.
Go to the boat in the south and board it, waiting for Lu Su and Sun Shao to board it as well. With everyone on it, it will begin moving towards Cao Cao's fleet. There will be a few officers there to greet you when you land, including Cao Pi and Zhenji. After they have been defeated, start making your way north, taking out any and all officers in the way. Once you get to Cao Cao, defeat him to end the battle.