The game is pretty easy if you just get a bunch of AoE abilities. It seems that the damage / defense greatly depends on your character LEVEL and not really the skill tree bonuses. So the best thing to do is to identify an AoE for every character, and rush towards it. Once that's done, get the Rod / Int bonuses to maximize damage. For Fang just rush the 3 burst skills for boss fights.
Then just go to as high a floor as possible, and spam AoE skills (Sylphie's Quad Winder is easily obtained early) to level up. Rinse and repeat for every titan.