Characters LV limit is intended from the start, because the game is short and have no need for further exploration (after final boss is defeated). I have finished balancing the enemy, although the outcome still not balanced enough. Maybe if I develop this game again I will make the LV limit increase based on how far the story goes (assuming this game is still in "Chapter 1", LV 20 limit is enough, I think).
I will think more about this LV limits. Thanks.