Based on the few threads I read, the dragon may swoop down to try and hit you with its claws. Maybe that's when you slash it.
There may also be the possibility of sneaking up on a dragon when it lands for whatever reason, but this method is unconfirmed.
You won't use ranged attacks, surely you will use sneak?
Yes, you can slash at a dragon when it swoops down, I believe it was in the trailer or demo? Not sure, but I would say its basically confirmed. The dragon likely won't just sit up there in the sky and shout at you, it wants to kill you fast, and it has big, nasty claws to help with that.