[Boromir88]

As Gandalf might say, here follows an account of general info pertaining to Balrogs...

Externally, just looking at it as a story, Balrogs are meant to be a match for the Eldar as Dragons are a match for Men. Balrogs were more than captains leading Morgoth's armies, they were his on-field commanders and highly skilled (plus powerful) combatants. They are meant as a mighty strong opponent for the greatest of the Eldar (or maiar in Gandalf's case). As Draughohtar mentions, they drive off Ungoliant, they slay Feanor, Glorfindel and Gandalf battle Balrogs. Men's "great opponent" are the Dragons. For this match to work, from a story standpoint, dragons have to be slightly less powerful than balrogs, as Men are less powerful than Elves.

Of course there are situations where any sort of 'luck' or certain circumstances can lead to a lesser opponent defeating a greater one. There are also Men who are more powerful than Elves, but in a direct comparison the greatest of the Eldar are superior to the greatest of Men, the average Eldar are greater than the average Men. I'm just talking generally here, in how Balrog, dragons, Eldar, Men...etc are presented in the story.

Now within the context of the story, Balrogs being incarnate could of course be slain and in some hypothetical scenario of "How many dwarves does it take to kill a Balrog?", of course tens of thousands could feasibly do it (that would assume the dwarves don't flee in terror from the aura of fear and power a Balrog can command). But seeing as they were Morgoth's on-field commanders, I doubt a single Balrog would take on tens of thousands of dwarves. If they were vain, reckless idiots, Morgoth wouldn't have used them as captains. They willingly seek out battle and challenge the greatest of Eldar (plus Gandalf), but they also show tactful retreat when it is necessary. They do fight pretty dirty but by no means are they reckless or cowardly.