You can waste a good deal of time and money fighting it, and then still get garnished in the end.
I probably get this call at least once a week. The potential client has a lawsuit that was given to their 12 year old daughter by a process server (who does know better), and they want to fight it. Yes, they owe the money, but they hope that there is some kind of law that will invalidate the underlying debt and give them huge monetary sanctions against the creditor and the process server. It doesn’t work like that.
You can hire a litigation attorney to argue improper service, and after spending about $2,000 on that defense, the creditor will simply serve you the lawsuit in the hallway of the court right after you beat him on this issue. You still owe the money, but now you’re out another $2,000 for fighting the admittedly improper service.
So yes, you can fight it and win a moral victory, but you still owe the money, and they will eventually be able to garnish you.