It would be possible, but very hard. Let's put it this way, if Elminster's tower could be destroyed by a Wish, someone who have done it by now. Think massive spell-battle, with the best casters of the Zhents and some Bannites on one side, and Elminster, maybe one or 2 other Chosen of Mystra, and probably a bunch of other adventurer types who were just around when the fighting breaks out.
However, the risk is so great that I doubt anyone but the truly insane would try it. Let's say the Zhents win, killing or driving off Elminster, destroying the tower, and siezing control of Shadowdale. How many powerful spellcasters did they just lose? How much gold did they spend to organize the attack? How many powerful people did they just upset? There's no way they could realize any long term gains because they'd draw so much fire from both Elminster's friends, opportunist groups like the drow, or maybe Sembians. I'd imagine that some red wizards might drop in to kill some Zhent mages while they're out of spells, or the Shades might start swooping in.
Big FFA battles between the Realm's heavy hitters are going to be too risky for most people to enjoy, and will draw too much unwanted attention for the winner to make good his escape. That's why they rarely have major battles and why the PCs have things to do.