It is helpful to distinguish some different categories of defense or claims.
In response to a mortgage foreclosure complaint, you might interpose certain basic defenses. For example, in some instances, a defendant might deny the existence of a valid debt or promissory note or deny that there had been a default under the alleged note.
A general denial, where permissible, tends to traverse all of the plaintiff's accusations. In the Rules of other jurisdictions, a defendant has to expressly deny particular allegations and even to admit so much of the allegations as are actually true.
An answer might also include some special defenses. For example, Mr. Roper has explained in another thread that failure of conditions precedent is a special defense and the denial of an essential element of a plaintiff's cause of action.
Other defenses are termed affirmative defenses. These defense are characterized by a defendant bearing the burden of proof on these points. Payment (the borrower paid the debt), accord and satisfaction (the borrower and lender previously agreed to some settlement of the debt), discharge (the lender forgave the debt), limitations (a statute of limitations bars recovery), and res judicata are all affirmative defenses.
With an affirmative defense, usually the character of the defense is such that even if the plaintiff proved the allegations of its complaint, a defendant might avoid liability and a judgment by proving the elements of that defense.
Separate from defenses are counterclaims. A counterclaim is a claim by the defendant against the plaintiff which arises out of the same transaction, occurence or set of facts.
Usually, the defendant bears the burden of proof as to counterclaims. Success on the counterclaims may establish a defendant's entitlement to some requested relief, but may not actually negate or excuse the borrower from responsibility for the plaintiff's claims (when proven) as would be the case with affirmative defenses.
Suppose that A steals B's bowling ball and drops it in the harbor. B learns of the theft and confronts A, beating him with a baseball bat and causing A's hospitalization with severe injuries.
A sues B, alleging injury from the assault and seeking damages for his medical treatment, as well as pain and suffering. B files a counterclaim for the loss of his bowling ball.
As can be readily seen from the example, a court is unlikely to excuse the assault based upon the theft and destruction of the bowling ball. The counterclaim as to the theft of the bowling ball is hardly either a defense or a special defense to the assault complaint.
If B prevails on his counterclaim, perhaps he can get the amount of A's assault judgment reduced by the cost of the bowling ball. But this is really not a valid defense to a cause of action for assault.
By contrast, suppose that B attacked A and A defended himself, but that B was injured in the altercation. It seems far more likely that "self-defense" would be a valid defense to an assualt complaint. That is, A will answer that he didn't actually assault B at all, that it was B who had initiated an unprovoked assault. A was merely defending himself and B was injured as a result of B's assault. A is denying the assault as alleged rather than seeking to justify an assault.
These are probably not very good examples and a lawyer could probably provide better. But the idea is that you need to distinguish between issues that are valid defenses to a cause of action and those that might support counterclaims or cross-claims.
Bill's point is also a good one. In my example above, making a counterclaim for the cost of the bowling ball might simply antagonize the judge or inflame the jury. That is, the cost of the bowling ball might not warrant getting drawn into the fray or testifying. If B really wants to press the issue of the bowling ball, he is very likely going to make it seem that he believes the attack was justified rather than asserting that he was remorseful. Any assertion that A had it coming due to the theft of the bowling ball is unlikely to be persuasive.
Bill's point seems to be that you need to have an overall strategy. What was described in the original post doesn't seem to reflect a strategy, but rather only a grievance which might distract from rather than support a good defense.