This is actually a pretty easy trophy if you do it right. What makes this easy is you can use a custom scenario!
So first you want to go to Edit mode and make a new custom scenario. For this scenario you will want to make only 2 kingdoms. Your's which will consist of every territory but one and a second Kingdom that only has one territory. For your kingdom you are going to want to load it up with Powerful officers such as Lu Bu, Guan Yu, Zhang Fei, etc... For the other kingdom you will want to put only 2 officers, and they should be weak officers that you can defeat easily, such as non player characters. Now go ahead and save this scenario
Now that you have made your custom scenario you will want to go to Empire mode and select your custom scenario.
Now before you start you want to set the difficulty to chaos and hit R1 to select your bonuses. Make yourself level 50 and give yourself 50K of each resource, You will need it for the troops.
Now you can sit back and let your officers do this for you or you can help them out if you like but be warned the enemies are strong.