I've been sued by a debt collector and I got out of it. It only worked though because I had a legitimate reason not to pay.
A car rental company (AVIS) double charged me and every time I called customer support I'd be connected to sum nigress who wouldn't listen to a fucking word I said and would just repeat
>"Uhmm but it says right here you were supposed to pay"
>YES I did pay, but I was double charged. I only backcharged the extra charge. How could I possibly return the same rental car after a week rental, twice on the same day?
>"Uhmmm but the receipt right here says you need to pay?"
FUCK AVIS.

So after a few months of back and forth they sold the $750 collection to a collections company. I received a letter from the collections company saying I have 30 days to contest or they'll start calling credit rating agencies. I collected every email, bank statement, credit card receipt, and piece of documentation and I typed on professional cardstock a concise 1 page summary of the events and how I attempted to resolve it and mailed all this documentation in.
A week later I received a letter from the collections company telling me they were no longer pursuing the debt.