Only attorneys can give legal advice.
However, if I were in a similar situation, I would consider composing and filing an answer in a "paragraph-by-paragraph" response to the complaint and then use the Discovery process (Request for Admissions, Interrogatories, Production of Documents and Depositions) to "discover" what evidence of a debt the Plaintiff plans to use against me in court.
There might be a chance the Plaintiff doesn't have any evidence that meets the lawful definition of "evidence" as required by a State's "Rules of Evidence". If the Plaintiff is unable to present the court authenticated evidence of a debt, the case "should" be dismissed upon the Defendant's motion to dismiss the case.
An excellent source for "general" Rules of Civil Procedures explained in a chronological fashion, is
O'Connor's Federal Rules - Civil Trials by Jones McClure Publishing.
I hope you find this information helpful.