Thank you guys very much for the advice!! I had achieved Martial Artist on chapter one in the catwalks, so I tried to get Feral Fighter in the same place, over and over. After consulting this thread, I exited the game and tried Rorschach vs. Nite Owl...but AGAIN it wouldn't pop.
So I exited to dashboard, trid RvNO again, and it worked first time! Cheers!
"I got a fever. And the only
prescription is: MORE COWBELL!"