Does your opponent have an arguement (then counter it), if your opponent is using observation in lieu of arguement, that is rhetoric and needs no rebuttal.
The first party of a debate that resorts to use of ad hominem, by default is signaling capitulation at best and at worst total defeat.
Credentials matters not in the least, unless the person can be argued into a corner and they need to get out of the kitchen because the big boys are playing hardball. Get my drift? And no, before you even inquire, rest assured that I'm not a forensics captain either, and that I could very well be wrong (I'll admit when I am).