Smaugs biggest weapon is his fire breathing......that would simply make Balrog laugh.
I assumed fire would be negated on both sides.
Claws, rows of razor sharp teeth in jaws the size of a small car, tail, wings and armor.
Beyond genius level intellect, magical abilities (at least in D&D), cunning, viciousness and vindictiveness beyond most other entities in ME.
Size and strength.
The only way Samug could have been defeated was a 1/1,000,000 shot.
Once he took Erebor no dwarf, elf, man, orc or wizard went near it for almost a century untill they knew he was dead.
I'd say the majority of Balrogs fell to weapons in the hands of elves (possibly some men), Gandalf with an advantage with being not only a wizard but also wearing a ring of power.
In a one on one Smaug definitely has an advantage.