If Elminster used all his resources (which he probably should, if he took on the likes of Larloch, Szass Tam or Shaaan) -- including Spellfire, his abilities/powers as a Chosen (including Silverfire) and a vast array of powerful/Epic magical items and artifacts. I'd hardly see Delayed Blast Fireball or Improved Invisibility working with Szass Tam, for example -- in fact, even mid-level Zhentarim magelings or Red Wizards should have 'See Invisibility' and resistance (or even immunity) to elements (fire and lightning, at least). And, as Ed Greenwood has often noted, an "epic" magical duel will cause permanent damage to the Weave -- Elminster and the other Chosen clashing with Larloch would probably cause as much environmental/collateral damage as dropping a small nuke in that spot.