Wow, this has actually been a really fun thread to read, so for what it's worth here my two cents.
Number one, the BBEG is perfectly within his rights especially if he knows this final confrontation is coming in finding his own GS. It's actually a fairly common trope in movies and fiction. Example, in "Blind Fury" the BBEG had spent most of the movie watching his minions getting taken out by a swordsman, so he went out and hired the best swordsman he could find for a huge final confrontation. In my honest opinion completely reasonable, regardless of what the player might think, and that is exactly what I would tell the player when he starts to raise a stink.
Number two, fire is your friend. People have been talking about how wall of fire will break line of sight but nobody's mentioned the most obvious thing. I don't know about your world but in this one gunpowder is extremely flammable, and frankly anything that's hot enough to do actual fire damage is hot enough to explode gunpowder. Now if you have something that's hot enough to do actual dice damage and is also an area effect attack, especially something with a duration like oh say a "wall of fire" I for one as the GM would start making any player dumb enough to get close to that start making some seriously difficult saving throws for his equipment. say like DC 15 plus the damage dealt so if the GS were to get within 20 feet were talking DC 17ish, within 10 feet about a DC 20, and if he were to try to actually move through the wall of fire... DC 30 minimum and that is if you even let him after he was stupid enough to try to walk through a wall of fire carrying probably quite a lot of gunpowder on him at the time. Or hell, we are talking about an Efreeti, how about if he targets the guns themselves with those quickened scorching rays, can we all say boom!
