Nothing specific you as a player can do about unlocking this. The maps are on rotation so you simply have to wait for each map to come up in turn. The only thing you can do make it unlock faster is win!
Capturing flags earns big points, 30 for cap, 10 for assist. And you get points for defensive kills. If you want to get this quick avoid the battle and cap and defend flags best you can. Which will also get these out of the way in no time-
Solution for Veteran in Battlefield 1943Solution for Defender in Battlefield 1943