Start a game as Haesteinn of Montaigu in 867. Make sure the game is on ironman mode and achievements are obtainable.
As soon as the game begine declare war and attack Brittany. Your army is vastly superior so as long as you don't give them time to form alliances you should defeat them quickly and easily.
You can now form the Duchy of Brittany, which will promote you in rank and unlock the achievement.