Seems like most Greyhawk wizards could mop the floor with Frealms denizens, but he would be particularly hard to beat since his player literally wrote 90% of the grimoire (along with all the other rules.) If nothing else, he could chunk Mordys Dis around until Elminster were no more than an anemic fighter with an AC that looked like first level THAC0 (and vice versa.)

Of course, since Greyhawk and Faerun are separate worlds, Mordenkainen can never confront Elminster, which is the biggest problem with "who would win a fight between..." debates: The very premise is usually so implausible, if not impossible, as to moot the discussion; it does not matter who would win a fight that can never be. Worse, the nature of the hypothetical battle between fictional foes precludes resolution of both battle and debate. Elminster throwing down with Khelben is even less likely than Egwene and Elayne flinging balefire at each other, or Mat and Rand dueling to the death. Rand COULD conceivably end up in mortal combat with Gawyn, and if Egwene (or Elayne, for that matter) is turned by a circle via Myrddraal, that could also happen, but even those are of dubious likelihood because of how they would impact the narrative. Any competition divorced from the narrative is divorced from relevance, making the whole thought-experiment diverting (or diversion,) but pointless.