This can be obtained at the very beginning of the game.
You will be in control of Leonids and to progress you will need to take down the 3 generals (marked with a crown over their level number above their heads). Click in the RS to lock-on, RB to attack & X to dodge enemy attacks.
After this is done, head to the edge of the cliff to initiate a one-on-one fight. Win this to unlock the achievement.