Try not to think "the only one who was", think "the only one that wasn't".

Rochallor- the only one who's fate we're not sure about?

And the thrush? :p Nope, your answer is wrong. :p

The thrush never met an elf?

Wrong.

Rochallor - the only one who didn't contribute to battle directly. Except probably to run away from trouble. :p

Nope.

The dragons that attacked Gondolin are numerous? :D

It's much simpler than that.

Simpler?
 

The thrush never lived in the First Age! Or Beleriand. Take your pick.

That's about the same level of difficulty, but it's not the right answer. ;)

The thrush never passed through Anfauglith!

No!

BIG clue: Fingolfin.