Always attack the enemy that poses that largest threat first. Then, focus on the less powerful opponents. Concentrating on healers is a good way to make sure that they cannot recover from your attacks. The CPU will often rest to get more mana for a character. When given the choice of a target, choose the one that has more mana, especially if they have just finished resting. This may force the CPU to rest with another character, taking them out of the battle for a turn.