In Batman Arkham Knight: A Matter of Family DLC pack, Batgirl has to destroy 10 balloons in order to unlock A Blade of Memory Trophy. Here are the exact location for the 10 balloons shown below in Maps.
Firstly, complete the DLC story mode and then in free roam you collect/destroy the collectibles. The Balloons are not marked on map by default so we recommend using the Detective vision to find them when you reach in one of the below locations. The Balloons are spawned in the outside area and not in any of the story buildings.
You can try the Batarang to pop the balloons instead of climbing up and destroying it. Once you finish destroying all 10 balloons…BOOM! you can unlocked the achievement A Blade of Memory.