Isonzo

Start custom match, make it friends only to make it easier on yourself, select 6th Battle of the Isonzo as the offensive and Carso as the starting level. Once you finish Carso, DO NOT exit the lobby and instead let the game load the next match, which will be Sabotino, followed by Gorizia. Once you finish Gorizia the achievement should pop. You're essentially playing the two levels back-to-back.