The typical response is outlined in my article and the accompanying materials that I have posted. Answer the summons indicating that you will appear in court, and do so before the expiration of the time limits. Send the plaintiff a request for production, even though they may choose not to comply. You can use that as leverage later in court, but it basically lets the plaintiff know that you are not going to run away. Instead, you are going to put up a fierce fight, and you want them to know that.
Yes, you may ask them for additional material, which is why I say to send them a request for production. And you are right, they bought thousands of accounts and have insufficient documentation to win their cases if you do your homework.