I believe I've found a way to reliably cheese BFG fight in Map 04:
Also after I have uploaded this, I've noticed, that the way Jacek Bourne won this fight in his blind run is the same as I did it blind - by doing an awkward unreliable AVJ. But what I show in the video (AVJ + Rocket jump) can be done consistently. Still, I'm wondering now, what was the supposed strategy for this fight.
Also sorry that it's so dark, I didn't know youtube butchers original video colors and brightness so much.