This is easiest done in a local multiplayer game set to two players. From the Main Menu, click New Game, then Select Players. Make sure your Gamertag is one of the players, and make the other player a Guest. To do this, click the red "X" next to the AI player name. Then click the "+" to add a new player. Scroll down to Local Gamer Profiles and select any guest player. This will allow you to control both players in the game.
Now, when you play the game, just focus on getting the most points possible for your player. Use the guest player to help your player get points (build bigger cities, longer roads, complete cloisters). You should have no problem getting this. This will unlock at the end of the game after the scoring.