When fighting against the world dragon or Acnologia, you will find (with a few exceptions), that the dragon is able to wipe out your entire team in one hit, no matter what their defensive stats. Because of this, Defense oriented characters are generally not encouraged. Try trading out your tanky characters for more offensively focused ones. You want a single, high burst of damage to maximize your damage against dragon, thus increasing the gold earned and the chance for potential rewards.
One trick that doesn't follow the above rules, however, is utilizing the card Mavis Vermilion. Because the way the dragon fight is set up, each of your characters will be able to execute a basic Magic or Attack based strike on the dragon, followed by their Nirvana. After your team launches its attacks, the dragon will then fire an AOE attack that wipes out your team. An interesting and very effective trick is to include Mavis in your team, as her Nirvana shields your entire team and allow you to get a WHOLE second round of basic attacks on the dragon.
Keep in mind that your substitution characters will NOT be able to participate in this fight, and their fetters wont work for your main 6 fighters. Find what works best for you, and good luck!