The final campaign – “Even Angels Cry” [7th campaign – 1st mission].

I can suggest leaving fewer units on the battlefield, because in reality, only one will end up tanking. (or leave only the strongest one, apply an aura, reinforcement, and regeneration).
Or, position everyone close together with guards, and apply an aura to them.
Or, heal everyone – if you have enough mana, prolong the battle until the resurrection spells heal everyone.
Or, deploy vestals, radiance, and unicorns on the battlefield – and then you can resurrect them in the city.