That dude was me.
If you want the exact way I did it I'll give you a quick step by step.
1) I welded up the door around the corner at the end of that corridor so that the Patty would likely come from the lobby instead of the stairs.
2) I planted 2 pipe bombs on top of one another just in front of that corner leading to the doors. They were placed so that I could stand around that far corner out of site of the patty and the pipe bombs wouldn't hit me.
3) When the Patty spawned I lied in wait around that corner for him to come to me. I didn't move to peek around the corner as if he can fired a rocket or sprinted at me I would be dead. I could hear him getting closer and when I thought he was close enough I bounced all of my Incendiary grenades off the opposite wall and around the corner at him.
4) As they exploded he stood on the Pipe Bombs setting them off. This was my cue to race round the corner and start spraying my Flamethrower at him at point blank range. He had already been dropped to 1 knee so I had a good 4 or 5 seconds of sustained burn.
5) He ran off to heal to which I immediately ducked back behind my corner, killing any zeds that came for me alhtough most happened to spawn behind the locked door behind me. I listened carefully for him again and when I heard him close I dashed round the corner again and sprayed with everything I had. By a stroke of luck I did enough damage in the previous encounter to immediately make him drop to his knee again and once again get a long sustained burn.
6) This time however he ran into the red light room to heal, which I rapidly followed. Just before he could recover I lit him up and killed him before he could reach me.
I didn't take any damage the entire fight. I kept saying for months that the Firebug was a very powerful class in the right hands, and this picture proves it. There was no tricks, no cheap pipe stacking, no Xbow, just a couple of pipes, a flamethrower and a Firebug who knew what he was doing.