This one is very easy, and not what the other guides are telling you.
When you have the "Battle of Guan Du" available, start the game in Empire mode. Don't do ANYTHING like assigning orders or anything else. Simply select Proceed to go to the Battle screen. Select Skip Battle and you will immediately be offered the special battle. Start the special battle, immediately pause and select retreat. When you get back to the menu, save the game and the achievement will unlock.
Other guides are telling you that you need to get to the rank of Count first. THIS DOES NOT WORK. I tried this a million times and never got offered the special battle. Just skip everything your first turn and it will be offered to you.
Also: Thanks to The Globalizer
for bringing up that you need to play as Yuan Shao to trigger the battle.