You can do this two ways: playing PvP or Co-op. I personally did this in co-op as I always play this game co-op (as the non-host, so I can confirm it still unlocks).
If you go for PvP, you'll need to be acquainted with the race, as the other player probably also is. An easy starter is as the Dwarves, you start with a big city (that has a big garrison) and with a reasonable income. Dwarves are always a easy starter race. As all human players play differently, it's just a matter of knowing how your race plays, how the enemy's race plays and getting lucky/being strategic.
You can also play co-op for this. Start a grand campaign with a buddy and just play through it as you'd like. As long as you win, you'll get this achievement. It you have trouble with this, try the lowest difficulty setting. You can still get the achievement on that difficulty, but it's really easy to win that way.
Which ever of the two options you choose, don't forget that the Chaos comes in from the north at a certain time (unleash the enemy player is playing the Chaos). So be prepared with enough armies, especially if you're Vampire Counts, Bretonnia or the Empire. With the forces of the Varg, Skaeling and Chaos combined, you're looking at one mighty enemy.
If you have any suggestions, please feel free to place a comment!