Who has managed to slay a Balrog? Gandalf and Glorfindel come
to mind right away, but there are others who have. Ecthelion
killed four, including Gothmog, the Lord of the Balrogs. Tuor
killed a whole mess of them. Allegedly, of course. Balrogs
started by being numbered in the thousands, though eventually it seems
Tolkien wanted to get down to just seven of them. (Figure, four
Ecthelion got, the one for Glorfindel, one for Gandalf, and that leaves
just one more for Tuor.)
Personally, I like the idea of tons of them, to varying degrees, some
with and some without wings, and all of them nasty, awful buggers.
For the most part. There's always that one really strange li'l
balrog who wants to strut about in a tutu and hug everyone. It
just happens. But mostly, evil and nasty. Nasssssty,
precious.
Not so lucky in their Balrog encounters? Pretty much, if you
slay a Balrog, it also slays you. However, there were some who
just got their asses handed to them by the Balrogs. Also killed by
Balrog attacks without slaying any? Feanor and Fingon. What
a way to go, eh?