Hey, I think I can actually help with this one... I took a look through the walkthrough pages (which you've probably already done...), and it looks like the dragon here is susceptible to Fire and Smash skills. (I also wrote that if you can raise your party's ATK or MAG, that will speed up the battle, so consider that as well, but focus first on Fire and Smash skills...)
So, look at the Class Progression List:
https://www.rpginsanity.com/crystareino ... sion-list/ - particularly at the "Bonus" and "Prereqs" columns.
Based on that I'd recommend you get Stacia to either Advanced Sorcerer or Sage, and Will to either Advanced Fighter, or God of Battle. (Depends on how much grinding you feel like doing - but probably the intermediate job classes will be enough.)
For the ATK+ and MAG+ boosts, I'd recommend to train one other person to Seeker for the skill "Swordology" (ATK+ for all allies), and the other one to Magic Fighter for "Evil Fighting Energy Wave" (MAG+ for all allies). You don't really need MAG+ for all, but you can learn that skill pretty early on, so... that's what I'd recommend.
Also, make sure someone other than Stacia has good healing magic (like Thaumaturgist "Cura" or Advanced Thaumaturgist "All-Cura.") You'll want this on one of your other characters, so that Stacia and Will can both just focus on attacking the dragon.
Let me know if this helps.
P.S. There's an item in the IAP shop that gets you extra Class Points, so leveling up the classes goes quicker... Depending on what device you're playing on you should be earning free IAP points for every so many battles or enemies defeated, and it should be enough to buy a couple of those items by the end of the game...