Healers will restore your party members' hit points. Allow them to do so at every opportunity. Keep Ophelia, the default healer, until she can be replaced with another hero. Strengthen her and do not use her as an upgrade for another person.
Skills
Queue up skills even if they are in cooldown. It will activate as soon as the cooldown period ends. Note: You can only queue up one skill at a time. You can also queue skills when the battles are in automatic mode.
Tankers
Tankers have high defense and lots of hit points. They are marked with a shield icon in the top-right side of their portrait. Place them at the front of your party.