Yeah man, well as far as I know, I have zero demo kills. I've played 36 hours. At that pace, I'll have the trophy in ... never.
Come to think of it, I've probably only been killed by demos maybe 3 or 4 times. It's just a dumb trophy all-around.
But I'm not complaining.
BTW: Is there a way to see how many you have? I might have more than I think.