WHAT THE HELL IS THAT THING!!!! I've come to the place where you first obtain the soul cube (I think), well I'm in hell anyway, and this huge demon has summonned. I keep firing my BFG at it, but it does jack all! Says somethin' about using the soul cube. I unleash the soul cube upon this thing but it don't do nothin'! Some help with this thing on how to kill it or progress past it would be nice and appreciated.
P.S. I don't want to use a skip mission/level/area cheat either. (If such one exists)