May I throw in a balrog?
"Ai, Ai! A Balrog has come!" Elves are scared.
"Oh crap, it's Durin's Bane." Dwarves are scared.
"What is this new devilry?" Men are either scared or curious. Probably both.
"Oh crap, it's a balrog of Morgoth. I'm in trouble now." Wizard is scared.
Since they can be destroyed, I count them as mortal, even though it would probably live until the end of time with no interruptions. Wielding terror alone, a balrog is enough to scare off most. It would be enough for me, methinks. Cloaked in shadow and flame, that takes care of a few more. And you can't say that using fear and fire doesn't count as fighting, because in a fight, you use the tools given to you, and that's what a Balrog's got. With flaming sword and whip, we've just taken out nearly everybody else. Throw in powerful magic and a heck of a lot of experience and most fighters are in big trouble. And the wings. Don't forget those. Any opponents will be so busy trying to document proof that the Balrog can kill them with no trouble at all.
Fea
How many wings does it take to get to the firey center of a balrog debate? The world may never know.