event Action EnemyKilled;
void Attach(Soldier observer);
void Detach(Soldier observer);

private bool isEnemyKilled;

private bool IsEnemyKilled {
    get => isEnemyKilled;
    set {
        isEnemyKilled = value;
        if(isEnemyKilled) EnemyKilled?.Invoke();
    }
}

public void Attach(Soldier observer)
{
    observer.EnemyKilled += BattleCry;
}

public void Detach(Soldier observer)
{
    observer.EnemyKilled -= BattleCry;
}

public class Archer : Soldier
    {
        bool IsEnemyKilled;
        private List<Soldirer> soldiers = new List<Soldier>();

        public void Attack()
...

public void Attack()
        {
            IsEnemyKilled = false;
            Console.WriteLine("Archer attack!");

            soldiers.foreach(soldier => soldier.BattleCry());
        }