Family Intervention Achievement
Defeat the Blood Brothers.Story-related; cannot be missed.
You will fight the Blood Brothers at the end of Chapter 6.
Initially, they will both be invincible. Once they "kill" you (you can't beat them), you'll get a cutscene and see how to hurt them: separate them so their armor deactivates, then unload on them. There isn't really much you can do to actually separate them aside from staying away, letting them charge at you, and dodging continually so they charge to different places.
You don't actually need to drain both of their health bars. Once you do enough damage the fight automatically ends and this achievement unlocks.