One
Balrog would be all that was required.
Balrogs are angelic beings, fallen angels, who do not have the limitations on their powers which constrain the actions of the
Valar or
Maia (such as the
Istarì) while within
Middle-earth.
In the
First Age, the only
elves capable of taking on Balrogs were the Noldorim who had returned to Middle-earth from
Valinorë, and were also at the height of their power. Plus, in every case of an
elf fighting a Balrog, it was a case of one of the mightiest elves, hallowed by Valinorë and unstained by the curse of
Fëanor, who happened to lose their own lives in the process.
Even
Gandalf, a fellow Maia was only able to destroy a Balrog by the sacrifice of his own life in the process.
Only
Sauron was a real equal to the Balrogs (and then, by
the Third Age, only with The
Ruling Ring, as he had lost several bodies in the interim, and the Ruling Ring helped to amplify what power remained).
So, even though
Dwarves were heartier than were
men, they were no match for even an Elf, much less a Balrog.
Add to this that they had to fight the Balrog within the confines of
Moria, where it would be difficult to bring numbers to bear, and the Balrog is at an even greater advantage.
The Balrog of Moria was probably not even as strong as it was in the First Age, as, like the Elves, the passage of time and fading of their power occurred with the passage of time.
One of the common comments regarding the Balrog is its relationship to Sauron.
Sauron was
Morgoth's chief Lieutenant, but the Balrogs were a sort of personal Guard of Morgoth; his special shock troops (their numbers remain unknown, with
Tolkien stating later in his life that there were probably only seven - nine Balrogs... Including Gothmog), and thus it is likely that without Morgoth around to maintain the hierarchy, that the Balrog of Moria might not have acknowledged Sauron's suzerainty and authority. No doubt the two would have worked out something during the
War of the Ring, had Gandalf failed to destroy the Balrog (his real reason for going through Moria - he knew that he absolutely had to destroy that Balrog, or there really would have been no hope of defeating The Shadow).
But as for the Dwarves.... The Balrog needed no help to take Moria from
Durin's folk. He (the Balrog) is a manifestation of The Shadow falling over Durin's line as a result of the presence of one of The Seven Rings by Durin's line causing their greed to overcome them. He is their "wages of sin" so to speak.
MRB
I obviously take Middle-earth too seriously!
