[Keeper of Dol Guldur]

How does one kill a Balrog, I mean, in Gandalf's case we realize that he fought it 'tooth and nail', with every thing he had physically (and probably spiritually too), hacked at it, and weakened it, and though it sprang anew upon the tower of Durin, it had to be weakened to the point at least, where if it were winged (I'll try to avoid that argument by saying 'if'), it was too weak to fly, and fell crash bang a few hundred feet. As for the depiction of Ecthelion stabbing Gothmog with his pith-spike, and dragging him into the water of the fountain, it's possible. The Balrog could have turned to slime (I'd imagine something like 'Morpha' from the Legend of Zelda, Ocarina of Time), and still attacked, who's to say Ecthelion didn't continue to hack away with whatever swords (or his helm). Of course, it can't possibly be an entirely accurate account of Ecthelion dragging him down, if the Balrog was twenty or more feet tall, but we won't rule it out-it's the only idea in existence. As for Glorfindel, the Balrog sprang upon the group, and he must have been riding foremost, because he managed to get to the thing before it wiped out everyone on the narrow pass, and whether he tripped the thing, or struggled to fight it, or used its whip like fiber wire, choked it, and it fell off the cliff, we'll never know, but we know that he didn't fall, they (Tuor and the folks) buried him under a small pile of rocks on that very pass. If Gandalf's Balrog (which was still full strength), had enough time to cast its lash up and pull him off the bridge, what was stopping Glorfindel's from doing the same, unless the Golden-Haired warrior kicked the crap out of it first (a difficult task on that narrow way).

On the note of the weapons used. They were all from Gondolin, Glamdring later slew Durin's Bane, but what of Orcrist? Glamdring survived a Balrog hack-fest (without melting or Nazgul-style dissolving), so did Orcrist as well? Was it the blade of Ecthelion, knocked away by Gothmog so that Ecthelion had to use the only thing left-his hat-to kill the damn thing? Maybe, but even so we know that Ecthelion had a Gondolin-made blade, most likely, and Glorfindel probably did too (and no doubt Glorfindel fought a skirmish on the mountainside).

So the great falls worked, but with the evidence comparing Gandalf to Glorfindel, they needed to be weakened to some degree. Whether or not they could have 'flown' away from it, could severely have influenced the need to hack a few fiery chunks out of them first, and to exhaust them to the point of not flying (or just landing on their feet or something amazing like that). Stabbing them alone wouldn't work, but stabbing and drowning did. Durin's Bane became a creature of slime, but slime usually floats on the surface and has some access to the air, while holding a Balrog underwater would likely drown it. Are there other ways to smite a Balrog? Not that I know of. Poison wouldn't work on a thing made of fire within. Mere weapons, nope. One of great enough spirit may have been able to wrestle it mentally (or spiritually, etc...) and defeat it, from within, sort of.