Well i just got to new area and holy....its so big. Also, its not realy a problem to lvl up classes as some areas you can just visit when exploring off the quest and kill monsters there for those lvls. Also for unlocking specific class, just have a hero of that class in party till you get 2-3 characters that can use it.
And here is another problem. Some characters suck at specific class and its best to just lvl them up with nopon coins (those Ranked D/E) to get the specific skills.