For me, this was the last map that unlocked in Empire's mode. If you just want to get this achievement and you are not looking to get the two battle scenario achievements in here, this would be the fastest way to get this achievement:
When you are setting up, choose Experience, this lets you use all the exp that you gained throughout playing the other scenarios as long as you were doing the same on them as well. Have max officers, annex the ruler, novice setting, just put things in your favor.
This is what I did and then I chose Cao Cao becuase he already owns most of the area. He is also the best set up to take out the other rulers in a quick amount of time. You can also still trigger the battle of Chi Bi battle scenario if you pick Cao Cao and skip the first round. I triggered it twice and I also overtook one place on the top of his area that was unoccupied just to see if it would work.
Now if you were playing with Cao Cao most of the game or with anyone on his team, they will still have the stats that you got them from earlier on. So you will be able to run through this play through very easy on. Now, this will not help you get the battle of Si-Ling scenario. You will have to play as Wu or Shu for that.
If you did not save your Exp and just restarted on each level, you are better off going after the two battle scenarios that are within this map so you can finish off this game.