- Put points into Piety and/or Vitality at level-up to increase it.
- Have one of your casters learn the "stamina" and/or "rest all" spell(s) so they can be cast during combat.
- If your bard is female, there's a necklace of endurance available in the temple in Arnika that regenerates stamina.
- Much later in the game there's a robe of rejuvenation that does the same.
I might have missed a few but those are the primary things to help your problem.