This game is really fucking hard. I bought it on Tuesday and it's as demanding as a Civilization game so far. Very fun, don't get me wrong - but fuck if this game isn't tough.
Oh, and the way NIS games are balanced, trying to keep your party balanced is counter-productive. Use one character, who will eventually becoming strong enough so that you can't afford not to use that character, thereby increasing further the amount of EXP he/she will soak, and ultimately creating a monster capable of tearing through the entire enemy force alone.