You can get this achievement by doing the following:
Set up a Multiplayer (Online game) with the settings of Deathmatch with atleast 2 Human Players and 1 AI (You need 3 players for this).
Load up the match and click the following:
Now select the human player and you should get the following screen:
Click Yes and then end your turn. The other human player will now get a screen which gives you the option to accept or decline the alliance. Accept the alliance and end the turn.
Now however is going for the achievement needs to break the alliance by going into the same screen above and repeating the process or by attacking the allied player.
This will work over Xbox or aSync multiplayer :).