I found out about this case when I got my pay check last week with a garnishment on it.
I was never served any papers, they were served to my 90+ year old previous landlord - I moved out four months before he got the papers. There are two other cases since then that have been dismissed because they could not serve me.
You will need to file a motion to vacate the underlying judgment. I would start by getting a complete copy of the court file which will include the proof of service. Then consider either an affidavit from your former landlord if you are on good terms or other proof documents that evidence where you were living at the time they served your 90+ yr old previous landlord. If you have all of your proof documentation ready to go, I would suggest you hire a lawyer to help you vacate the judgment. If you win, you should be entitled to an award of reasonable attorney fees for prevailing on the motion to vacate. Once you file papers to vacate the underlying judgment, you should be able to suspend the garnishment pending a ruling on the motion to vacate.
You will need to file a motion with the Court to have the jdugment vacated for lack of proper service. That will stop the garnishment temporarily, although if the statute of limiation hasn't run you can expect that the creditor will continue to seek to have you served and you will ultimately need to address the debt.
If you don't have an actual legal defense to owing the money, you may want to consider negotiating a settlement or consulting with an experienced bankruptcy attorney.
You cannot get it dismissed as there is already a judgment against you. You can try to get the judgment vacated because of bad service of process, but that will not be successful for a couple reasons. First, you DID live at the address where the papers were served, and serving them upon someone at that address is probably considered good service. Second, even if you do get it vacated, they will simply sue you again, obtain another judgment (and judgments on the others that they couldn't obtain serve), and re-garnish your pay. Your time would be better spent trying to work out a settlement since you don't deny actually owing the money. Don't waste your time litigating this - you'll simply end up losing anyway unless you can actually deny owing the money.