The only way I stopped the dragon from killing the goddess in the first turn (before I even act) was to make sure that all my characters were as far up towards where the dragon appears as possible when the first battle ends. Then the dragon would focus some of it's attacks on my guys and the goddess would live.
As far as using any spells against the dragon, I call BS. I tried everything possible against the beast and it's immune to everything or the spells simple fail. Either way it's a waste of AP to even try. Ended up killing it by hacking away at it for an hour while one character healed and another took care of the summons.