There are actually quite a few defensive pleas available to a suit on a credit card debt by the current owner of the debt if the facts exist to support them. The problem is raising them in a motion for summary judgment is a very technical process and you will not be able to do it without an experienced lawyer. You would also have to amend your answer to be able to raise the defenses in a response to a motion for summary judgment. There are some very strict time deadlines that apply under the rules of civil procedure as well. You are in over your head but with an experienced lawyer you might be able to defeat the motion.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
This is not the space to answer such a detailed question. Spend a some money and hire a civil trial lawyer to defend this suit. If you don't get professional help, you'll lose and deserve whatever the result is.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
To answer your question directly, what defenses you can potentially successfully assert depends on the factual details of your case. Since you do not say what those factual details are, I cannot tell you what defenses might work.
About all I can say for certain is that I agree with the other lawyers who have responded by telling you that no matter what your facts are and no matter what you try to do in the lawsuit, you will almost certainly lose unless you hire a lawyer. Responding to a MSJ is technical and complex. Unless it is done just right, following the applicable law and rules, the judge will be required by law to grant the motion.
There is also a strict time deadline for filing a response to a MSJ. If you fail to meet it, the judge will not consder your response. Therefore, I suggest that you consult a lawyer immediately.