This achievement can be earned in Skulls Online (on XBLA) or Skulls Anywhere (all devices). Create a Deathmatch game with 3 or more players (only two of whom need to be human, the rest can be AI). Once both humans have selected their unit color (which is the start of the second human player's turn if using Skulls Anywhere), have the second player open the Info screen (press on XBLA, tap or click on the tan scroll icon on WP and Win8) and select the other human player. Choose Form Alliance to create a Deadly Alliance. The other player will need to accept the alliance on their turn, and at that point whoever is getting the achievement will need to open the Info screen and break the alliance. The achievement unlocks immediately.
If synchronizing across multiple devices, you must load the save game on all devices simultaneously for the turn in which you are going to break the alliance or you will not be able to utilize the same save for this achievement.