Regarding the WK’s power at the Pelennor Fields as compared to earlier in the book – Tolkien mentioned in one letter that:
So no, the Witch-king who was driven off by Aragorn with a burning stick was not quite on the same level as the Witch-king confronted by Éowyn.
Regarding Balrogs: I seem to recall that Tolkien changed his mind a bit on exactly how powerful they were meant to be. It’s true that in earlier versions of e.g. the Fall of Gondolin many Balrogs are said to have been slain by Elves. But sometime later, I seem to recall (I can’t remember exactly where this was said) that he was contemplating making the Balrogs much more powerful than they had been, and much rarer (i.e. instead of there being hosts of Balrogs under Morgoth’s command, that only like seven of them had ever existed). So it’s not at all certain that the Balrog Gandalf fought was conceived of as being at all the same order of creature as that killed by Ecthelion, Glorfindel, etc.
…And a mace the size of a U-Haul. (IIRC, Jackson got that right but changed it from a scepter-like mace to flail. Which was awesome.)
One thing to remember is that the Witch-King had not much less mystical knowledge than Gandalf and far fewer compunctions about using it. He wasn’t “just” a shade, either, but a fell spirit an eon old who had studied at Sauron’s armored heel. In a magical duel, it would be a difficult duel. And given their physical abilities (neither one tires without at a minimum, one very long fight) it wasn’t going to be over quick.
I still give it to Gandalf, but not by much and not a certainty. It’s not certain that Glamdring could have dealt the WK a mortal blow, and in general magical powers in LotR were more used to weaken or incapacitate rather than directly kill. Both were incredibly tough and skilled combatants along with the knowledge and neither goes down easy. But that was in keeping with Tolkein’s tendency in combat scenes - nobody was "safe and being stronger or faster or more skilled was an edge, but not a certain one. No battle was safe even if you outclassed your opponent, to the point that one well-equipped elf lord dealt Satan (more or less) a permanent injury.
I’ve never taken Glorfindel’s prophecy to mean that a man couldn’t kill the Witch King, but merely that a man wouldn’t. “Not by the hand of man shall he fall” != “No man can kill me”
Gandalf “wins”, but maybe not in the sense of a mano-on-mano confrontation.
Here’s my thinking: Gandalf is a Maia, a being much greater in origin than the Witch-King.
As an Istari, Gandalf is limited in the power he can manifest in Middle-Earth, but the With-King is only a human who has grown great in sorcery and then only retains that power as long as the One Ring exists.
It’s been a while since I’ve read the works, but has there been an example of a Maia being destroyed by a non-Maia without mutual self-destruction? Luthien rang Sauron’s bell (and even Morgoth’s in a sense–yay lady), with help, but did not destroy him. Elves who managed to kill a Balrog also died in the process.
My own pet theory as to why Sauron grew to be so terrible, and why Saruman or Gandalf would only be a hawk under the eagle’s talon is that practice makes perfect. Sauron long ago shed his restraints on exercising his power in ME and learning sorcery to the extant the Istari could never match.
All may have been Maia in the beginning, but the guy who has a millennium of batting practice is going to hit the ball better than the newcomers.
I think that Sauron was always greater than Gandalf (and most of the other Maiar, for that matter) in terms of pure raw power. Gandalf, however, is explicitly stated to have been the wisest of the Maiar.
I think it’s implied that Luthien and Huan could have destroyed Sauron when Huan had him pinned down, but instead they just made him give up control of his tower. But then Luthien is “half Maia”, and Huan is… who knows what.
I don’t have access to my books right now but there is a piece where the Valar discuss sending the Istari. Alue chooses Curumo (Sauruman), Yvanna picks Aiwendil (Radagast) as balance - which goes over about as well as you would expect. Manwe selects Olorin (Gandalf) to go but Olorin initially declines stating that Sauron frightened him.
Tolkien said contradictory things on Sauron vs Gandalf. In some places he suggested they might have been equal in the beginning, while at other times he said that Sauron was always more powerful. I’m not sure what his “final” word was on the question.
It’s been a while since I read any of the books, but yes, this is familiar and I would agree.
IIRC, Huan could have destroyed Sauron’s physical body and didn’t Luthien use that to get his surrender? Saying that Sauron would have to return to Morgoth as a “shadow” and face his scorn? Or, something along those lines?