This is a story-related trophy that cannot be missed via normal campaign progression.
You will earn this for completing Chapter 4: Opernplatz.
You start out by having to clear out a very large open area. There are a handful of snipers stashed in some sneaky hiding places, so keep your eyes open. After clearing the area, head towards the library. You will take refuge up there so you can halt the execution. Save your "target" and make sure he makes it to the library. Then you must hold off an assault by Russian forces.
Trophy will unlock at the end-mission screen.