The barbarians here are not a journalistic representation of a specific battle, just as the image of the decedent at the center of the scene is not meant to be a true portrait. The sarcophagus would have been commissioned by the family to position them as decendents of a defender of civilization from chaos and decay---an important sentiment in a time period when Rome was losing its grip on its colonies. The barbarians are characterized in a way which emphasizes their supposed inferiority to Rome and its culture, such that dominating them would be explained as doing them a favor, by "civilizing" them.
You can visit this famous Ancient Roman sarcophagus at the Palazzo Altemps Museum in Rome.
In this image, the decedent is represented as a glorious hero in a battle against Gallic barbarians.