Como se luta contra os Nazgûl?

Tolkien comments on this briefly in Letter 210. The whole letter is worth a read, since he scathingly (and quite hilariously) rips into a script for a proposed film version, but I'll only quote the relevant section (bold is my emphasis, italic is Tolkien's):

[The Black Riders'] peril is almost entirely due to the unreasoning fear which they inspire (like ghosts). They have no great physical power against the fearless; but what they have, and the fear that they inspire, is enormously increased in darkness. The Witch-king, their leader, is more powerful in all ways than the others; but he must not yet be raised to the stature of Vol. III. There, put in command by Sauron, he is given an added demonic force. But even in the Battle of the Pelennor, the darkness had only just broken.

^{_{The Letters of J.R.R. Tolkien 210: To Forrest J. Ackerman (Incomplete). June 1958}}

Tolkien suggests that martial might is not one of the strengths of the Nazgûl.

Amizade discusses why Glorfindel isn't concerned about them:

'The Elves may fear the Dark Lord, and they may fly before him, but never again will they listen to him or serve him. And here in Rivendell there live still some of his chief foes: the Elven-wise, lords of the Eldar from beyond the furthest seas. They do not fear the Ringwraiths, for those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power.'

^{_{Fellowship of the Ring Book 2 Chapter 1: "Many Meetings"}}

This "great power" is not elucidated upon, but it suggests that the Nazgûl aren't so much derrotado as "convinced that they're going to have to fight it out", and they don't seem to like to fight it out.

We must not forget that the idea of "power" in the Tolkienverse is not generally a físico one. In that, Tokien's beings have "powers" of medo, amar, desespero, esperança e influenciar.

As such, when comparing the beings' "power" levels, it is incorrect to think of it as fire "hurting" a Nazgul, but rather that fire does away with their advantage of discrição e medo.

Conseqüentemente, Glorfindel e Aragorn, who are unafraid and unwaivering in their own strength of amar, are able to withstand and repel the Nazgul. In these instances, it is not a case of "beating them with a vara sword"; it is a case of overpowering their advantage.

As you mentioned, Frodo crying "O Elbereth! Gilthoniel!" is a mark of both defiance against their "power" of medo, escuridão e mal, and Frodo's reliance upon the powers of amar, esperança e bondade.

This is also why the Army of the Dead were so effective in the battles against the dark armies, Como DVK mentions:

As you can see, their weapon[s] we[re] fear and terror, nothing physical.

This is a common theme in Tolkien's writings, a lot of the "magic" and "power" is not really physical in the sense that we might generally think of it and is often "off-screen".

Therefore, how can one man or a bunch of misfit (a man, an Elf, and four Hobbits) withstand these "mighty" beings? Well, simple, don't be affected by their "weapons". É por isso que o Witch King "resorted" to using the Morgul Blade contra Frodo, it was the only weapon that could have done the job (in this instance).

The [funny] case of Butterbur, Sam's elderly father (Gaffer Gamgee) e farmer Maggot são na verdade realmente good examples of this non-physical "power" which the Nazgul possess; the three não sabia what they were dealing with, and the Nazgul had their fear aspect "turned off" to be able to stealthily roam the North and ask about "Shire" and "Baggins"... Hence, the three men did not feel any fear and simply "slammed the door" in the Nazguls' faces and told them to "piss off".

How about grabbing a pointy object (like a pike, sword, arrow, etc.) and trying to peek the pointy bit into them? Or grabbing a heavy object (like a club, a mace, a battlestar or so) and repeatedly hit the nazgúl with it. AFAIK nazgúl gains their limited immortality - which is not invincibility - from their rings, and the rings have limited power. One only has to cause as much damage to the nazgúl as one can, and hope the power of the ring "dries off", so the nazgúl gets reduced to its semi-dead state.

As I understand it, they can feel some amount of pain hence them running after Aragorn burned them, and it is mentioned that no man can slay a Nazgûl therefore letting Éowyn kill the Witch king of Angmar. I imagine that Sauron would not have thought a mere woman could best the Nazgûl in battle.

This is speculation