You do not have to directly respond to the affirmative defenses. Many defenses are raised only as a formality to avoid waiving them. While you do not have to respond to them, if there is any substance to the defenses, you will have to overcome them to prevail. One way to do that may be by an affidavit in support for your motion for summary judgment. You will probably benefit from hiring an attorney who is familiar with the Civil Practice Act.
A sheriff's entry of service will usually overcome the service claim. If the case is with pursuing I would GET A LAWYER and have him move for summary judgment and attorney fees on this matter.