Dynasty Warriors 7

As an alternate method, you can play the first stage of Wu's story, the Yellow Turban Rebellion, part one (Or something to that effect).

You start in the middle of the battlefield and there are two officers west of you, and two to the east of you. The stage ends when you kill the 4 of them, and they aren't all that far from your starting point. Crank it down to beginner and kill all 4 of them and you'll get this with minimal effort.

Another alternate method to this achievement is the first stage of conquest mode with a friend, simply load up the level and divide and conquer in order to finish the stage up in under a minute. Should be easy enough assuming you are playing on a lower difficulty and have a decently leveled characters to use. Even so I believe there are only a couple officers getting in your way in this stage and with two people competing for kills this goes by fast. Simply rush the commander and the achievement is yours.

Definitely the easiest place to do this is the first stage in Conquest mode..."Qing Province Solo Battle". Simply just guide your horse straight to the enemy officer with the green target surrounding them on the map. Defeat them in a few hits and viola, achievement unlocked.