Each hero in Battle Breakers has a star rating which indicates how powerful that character is. One star heroes are the weakest while six star heroes are the most powerful. As you progress through the game you will unlock new heroes that you can add to your team. Make sure that the ones you choose are those with the highest star counts. You need to pay attention to your formation, ideally you should place melee units in the frontline and magic or ranged characters in the backline.