At least 9-B during prologue scaling from Burning Hands and able to kill intellect devourers and Goblins with ease. Quickly becoming at least 9-A, first few levels teach spells around that level + they scale from characters like
this child murdering ***** and
Auntie Ethel. Though, there is an
Optional boss as early as early as act 1 who reaches Low 7-B+. But there's a lot of unknown and speculation, some suggest if Grym should be
this level by lore. But can't be the judge of that.
By the end of Act 2 and beyond however, see
here for reference. And some could speculate endings reaching
High 6-A scaling from Vlaakith and/or Orpheus. Elder Brains appear to be in the
Low 6-B to 6-B range, but it does not look like there is a Nether Brain key (A more advanced version of an Elder Brain powered by the Crown of Karsus. But speaking of the Crown of
Karsus, I do not know the full story. But there seems to be more to it, but doesn't look like end game is any lower than High 6-A. Especially Gale's Netherese Orb or better yet the God of Ambition transformation; who is probably more or less comparable if not superior to
Mystra.