You can’t leave a team stronger than you found it. Heroes resurrected or healed in a battle will, at the end of an unsuccessful battle, be reset to their condition at the beginning of that battle.
Example:
Battle 1: Tank Aegir and flank Magni are killed; wing Alberich is at 50% health.
Battle 2: Alberich resurrects Magni and heals himself. At end of the battle, everyone is at full health and only Aegir is dead.
Battle 3: Magni goes back to being dead and Alberich at 50% health.