I did a second Ragnarok with Jorm Friend Eidolon this time. The extra -10% ATK reduced damage by a further 25%, making the fight pretty trivial. She was only hitting my neutral-element characters for 750 per normal hit, and 2200 or so for the Rage Overdrive. Even a triple hit from the Overdrive wouldn't kill my characters at that level of damage, and healing isn't necessary so I can save Sol's heal for debuff removal.
In short, stack enough ATK down and this fight becomes trivial. Some players might want to bring Balor to ensure debuffs stick if Mordred isn't an option. Sol or Zephyr are also recommended. Water teams should bring Jorm as friend eidolon.