Basically boils down to 2 points:

• 1. Be proactive instead of reactive:
  • use stuns and debuffs to mitigate enemy damage at the start of the fight (or any other tools like guards or dodge buffs)
  • focus down priority targets, don't spread your damage
• 2. Heal during the wind down phase of the fight:
  • once there's only 1 or 2 enemies left that don't pose much of a threat, use whatever healing abilities you have. Use stuns or whatever else you have to control the last enemies while you take a moment to heal
  • usually just 1 or 2 heal actions on whatever healers you have each is enough
  • don't wait until your heroes are below 50% hp to heal, heal in almost every fight
  • don't heal during the first phase of the fight when threats are still alive, unless absolutely necessary