Isonzo

Start custom match, make it friends only to make it easier on yourself, select Straferxpedition as the offensive and Cengio as the starting level. Once you finish Cengio, DO NOT exit the lobby and instead let the game load the next match, which will be Fior. Once you finish Fior the achievement should pop. You're essentially playing the two levels back-to-back.

I'll test this out with 6th Battle of the Isonzo as well and if it works I'll post the same guide there.