Avesta didn't actually solo Ash dragon the same way it would be needed to be done today. Avesta had other people killing the bombs in the room which would easily link and own anyone try to solo it without the help.
On another note, I've been able to solo Ash dragon to 20% using NIN/DNC but it takes over 2 hours and more concentration then any other solo I've ever done.
I highly doubt SAM/DNC could kill it, As soon as it uses Curse and your HP drops to 200/200 HP and that's 100% of the HP bar, 1 hit on the SAM and you'll be done.
The only jobs I know of that can solo Ash dragon are RDM/NIN, NIN/PUP, NIN/DNC, and NIN/RDM(Using the Enspell trick).