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.
Party configuration
Have a warrior (paladin subclass) and use their Taunt Strike to aggro enemies and soak up their damage. If you purchased the rogue, use it to damage to individual enemies. If not, use the mage in the same role and remember to have the paladin taunt excess aggro. Have a priest that has the single and group heals to keep the paladin alive.
Starting character
Select a powerful base class. The warrior (paladin subclass), as well as a powerful mage or healer are good choices. As you play, the souls will unlock subclasses for further customization. The premium rogue class can be purchased after you have an idea of how you want to proceed. Its two base attacks do physical and energy damage each, and also gain three mana. More advanced attacks will allow you to burn enemies, as well as increase damage or evasion.