It could be a new skill in combat. Revive or resistance.
First option revive: When a unit dies in combat there is a very small chance at level 1 to regenerate when the battle is over. The unit would be in your army with 1 point of life
Second option resistance: There would be a small possibility at level 1 that the unit, instead of dying, would stay at 1 point of life
Another option could be to increase the percentage of arc bonuses, 0.1 % for each level starting level 10 with 10%. Increase attack and defense by 0.25 percent for each level. Frontenac increase 1% for each level. Alexandria 1% for each level, etc...