He was powerful yes, but fair from the only powerful being in FR, and not even close to the most powerful, and at least point he's far from his peak of power.
Spoilers from Elminister from Hell below
Elminister got owned by an archfiend in Elminister in Hell, at least until the archfiend went too far. He had to be bailed out by Mystra, who basicly mind blasted predivinity Asmodeus.
Seriously Elminister has gotten he arse kicked and been in need of rescue before, suffered unspeakable torture in he'll,
much of it horrifically disguisting, had his body blown up, turned to ash, he most have top shelf PTSD.
Seriously no Mary Sue goes through a 10th of the torments that Elminister goes through, seriously it seems almost more divine punish then reward.
Even uber powerful Gods like Mystera and Shar lose big time in FR, and if any character has a right to be a Mary Sue it'd be Goddesses, but even they lose too.
Honestly most of the high rank heroes and villians function like Gods in the sense that they prefer to use minions then fight direct battles most of the time.
I find it funny that people are worried that Drizzt and the Chosen of Mystera was Mary Sues, when an angry God can send an avatar the weakest of which and beat Szazz Tam who is, who way more powerful then Elminister.
In 2e an Avatar of Sharess (5e terms Zandilar/Bast) was a 18th level Wizard/23rd level Rogue/18th level spellsinger/16th level cleric/10th level fighter with nine lives, could look like anyone via illusion at will, at will version of limited wish, and so on. This was back when they actually knew how to stat up gods like gods, instead of powerful monsters.
An avatar like the one above could have slaughtered the Chosen of Mystera and Drizzt and his Companions and an assortment of other FR heroes and Gillian's without breaking a, sweat. And Sharess was a simple Demigod, image what an Avatar of Sune or Bane could do. But people worried about Elminister. It's funny.