I have often stated that the primary hermeneutic is this: The Bible is meant to be read intelligently.
What that often means is that we are given general guidelines, but we are to use our brains when it comes to situations that are beyond the pale. The bible doesn't spell out every possible outlier. We should remember that one of the foremost ways that we have been made in God's image is that we are rational beings.
For example, the bible tells us to live as if we must reach everyone with the gospel. In fact we know that God, while pleased to use us, does not depend on us at all, and not one of his sheep will be lost due to my laziness or yours--although there certainly might be consequences for our sloth. God will have mercy upon whom he will have mercy implies that we may very well encounter some in glory who were saved in spite of never having heard the gospel. If you don't agree, you are telling God that He in fact cannot have mercy upon whom He desires, there is a precondition. But yes, we have the normative rule: live as if every single life depended on you giving the good news.
In the New Testament, the general guideline is that marriage is sacred. All effort should be made to preserve marriages. Forgiveness should be sought and granted. Extraordinary grace should be afforded. We should all live as if every marriage should be rescued.
This is all true. But it is guideline, not an inviolate law. We need to use our brains. If a woman is being abused she should not be encouraged to reconcile with her husband. Forcing a woman back to a man who has abused her is wholly inconsistent with the attributes of God. There is no example of such a practice anywhere in scripture. No where does it say: yes he beats you, or constantly berates and humiliates you, but he's your husband so too bad. The sacredness of marriage is that you persevere through hard times, you don't leave your spouse for a younger model, you don't leave because you need some space, etc.
Abuse is another matter.
There is a threshold. I'm not claiming I know what it is, but there is a threshold. To first order I'd say that any abuse exceeds the threshold--but let's just say that a threshold exists. And if there is a couple in a church and the man is abusing the woman beyond the threshold, she should be encouraged to leave him and divorce him. She should be loved and cared for, and the man should be excommunicated and treated as an unbeliever.
To often we accept the man's plea that he wants to reconcile and end up treating the woman as a victim. This post on A Cry For Justice describes a sad, common response that trivializes abuse.
I was a church leader--then I was demoted. But I tell you this: As a leader, when I had to make a choice, I relied on a trivial metric. If I have to choose between A or B, not certain which is correct, I would choose the option that would leave me less frightened when facing judgement. For example, in deciding whether a candidate was ready for the ordinance, if it was a tough call, I'd tell myself that I'd rather God asked me: "Why did you allow X to be baptized?" and not: "Why did you withhold baptism from X?"
To the point of discussion, I'd rather face the question: "Why did you encourage that divorce?" instead of: "Why did you coerce her into going back to her abuser?"