As soon as the ambush starts, keep an eye out for an ally that will be wounded by a point-blank explosion. Before pulling him out, make sure to create a smokescreen around him to avoid getting killed by enemy gun fire. Pull him behind the rocks nearby to complete the heroic action.
During the assault and while helping your allies are clearing the barracks, keep an eye out of an enemy rifleman (with the target icon above his head) that will attempt to snipe an ally. As long as you kill him first, the heroic action will be complete. Take note that his spawn position changes; it’s either he’ll be in the open or inside the barracks so you have to pay close attention to the enemy positions and the icon.