Split your troops into 2 divisions, train them while completing the war goal focus.
Hungary, same, you can take it as your first target.
Since the Bosphorus DLC they're completely nerfed and debuffed, there wont be any challenge here. You just follow the focuses til you get to own core and war goal against them. Now with Death or Dishonor, you get quite the ways to obtain core, mainly through focuses and decisions. For this achievement to unlock, you must have Core on at least one province of each bordering countries:
