I ran into one myself last night. I was hosting a server, with realism mod. I had the MG (I also have the blood patch to see my shots). Well this guy mowed down my entire team, and I was the only one left. I retreated to the church and waited for him. He planted the bomb, and as soon as he came out from behind the 88, I loaded him full of lead (I saw the blood and everything), however, it didn't seem to faze him at all, because he just layed me out like a doormat with his SMG.

I gave him 2 more rounds just to see if by some chance I missed him, but no one was able to kill him (not even his own team, I had FF on). So, I kicked him.