He took me 2 tries, but wasn't that difficult with the right strategy.

First, I made sure to blind the Ring Wyrm. And slow him, too.

I had Vaan with the Main Gauche + Golden Shield (Evade = 50) and manufactured Nethecite as well as always protected, so magic didn't do much to him and he evaded like 95% of the Ring Wyrm's attacks, and when he did get hit it wasn't fatal.

I had Fran with a bow and Fire arrows, plus a gambit so that she would cast berserk on herself. She was doing a good 800 damage a shot.

Finally I had Ashe running around and throwing heals and protects on Vaan.

When the Ring Wyrm was down to about 50% and I started being unable to keep up with the damage it was doing I brought out Balthier and summoned Belias. At this point the Ring Wyrm only cast spells, every third of which was a fire-based one, so Belias never died... while doing around 1500 damage each Painflare. After his Hellfire, unfortunately the Ring Wyrm still wasnt dead, so I ran around with Balthier until he had full HP again and summoned Belias again... yay Ice Sword for me at lvl 18!