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The best thing Cain could do to get ready for this quest was to level up the group he was going to take with him. There were no guarantees that the Bunny and Dragonkin were the avatars that the Quest referred to, but it was better to be safe than sorry.
Using the logic that the other Avatars should also be people who were close to him, since he got the quest, Cain decided to issue a Guild Quest, to turn in information relating to the laws that govern the world. That way as long as they tried, they were involved in the main quest and should gain the rewards.
At least in theory.
Getting Jin leveled up was easy, she was a Mage, so he could just send her to the tower repeatedly, or even to multiple towers daily to build experience.
There was also an experience bonus item enchantment among the abilities that he had learned through [Spell Crafting]. He hadn't had a chance to actually try it yet, but he had the diagram available.