Half of all abusive relationships are abusive on both sides, and even the remaining half are split evenly between male and female (slight statistical increase on the female instigator side but that's mostly due to a lack of intervention).
You may walk into a situation where the guy has the girl on the ground and in your head you assume she's a helpless victim needing to be saved. The reality may very well be that either:
They were both beating the snot out of each other in mutual aggression and he was simply more physically capable and both of them are aggressive, crazy, and likely to see your intervention as a threat to them going to jail or you taking away their ability to act the way they do.
Or the female attacked the male and the male is trying to restrain the female to stop the attack.
I was in an abusive marriage myself with a woman that developed a severe mental illness. I usually just curled up and took the beating and waited for her to tire. Any attempt to run away or restrain her just made the next attack worse. Had you intervened in my case I'd have probably argued in defense of my ex because she was mentally ill and couldn't be expected to behave appropriately.
That went on right up until one time she stabbed me in the chest. Thought it was just another episode, I might get another concussion in the worst case, and had no idea it was coming. So, with that in mind... I recommend a simple: "You guys need to stop." and call the police. Monitor the situation while you wait, and if it escalates to someone seriously harming the other (like pulling a knife or curb-stomping someone), pull the gun and order them to stop. Don't let anyone get killed, but give them a chance to break it up without shooting anyone, and if they both tell you to **** off, do so and let them handle it. Chances are they've actually developed a routine for this and no one's going to get killed.