Someone showed me this on twitter:

There IS NOT an achievement for beating Rivenroad (Hard). You CAN however see the timestamp via lodestone under "Legacy - Sixth Astral Era".

"Nariko Star has earned a place in the annals of history by besting Nael deus Darnus (Hard). - 10/20/2012"

I don't believe there is any way to confirm this strictly via game client though (I have not confirmed). And the person would have to have achievements set to public on lodestone for it to be visible but it seems it is at least documented.