[Carannillion]

I don't think Sauron would have needed the Ring to make Smaug fight the free peoples. A slap on the bottom and a 'there are riches beyond your imagining' would probably have set him off.

As for killing Smaug, maybe not ANY elf could find that weak spot, but a lot of them. Also, let's not forget Elrond here, either. Or Glorfindel. Elf Lords, one of them wielding a Ring of Power, the other just returning from Aman after having dealt with a Balrog...