Cuando suena el himno nacional

Unfortunately you need to own Bronya to get this achievement. Enemy Gepard and Cocolia can be found in Forgotten Hall 15 or Simulated Universe 3 and 6 respectively. Enemy Bronya can be found in Forgotten Hall 14 or Simulated Universe 6. Cocolia in the Echo of War does not count.

If someone says "When the National Anthem Rings," it's often followed by them telling you to stop what you're doing, stand to attention, and sing along one's national anthem. The Belobog March sounds like what the name of Belobog's anthem would be. Bronya plays it against Gepard, her mother, herself, or when Bronya sees that Seele is about to use her ultimate.