Originally Posted by Grizimus
I think you are refering to the guy at the end of level 4, I think.
You fight this guy three times, and he's the same each time. I can confirm it works all three times because I just did it on Hard difficulty. It takes a while, and you have to have good timing, but it definitely works. I just stood in front of the guy each time and spammed the right trigger over and over. It took a while, but down he went!