void OnTriggerEnter2D (Collider2D other)
{
        other.gameObject.GetComponent<Solder> ().HP -= damage;
        Destroy (gameObject);
}

public class BaseUnit: MonoBehaviour
{
    public int HP;
}

void OnTriggerEnter2D (Collider2D other)
{
    other.gameObject.GetComponent<BaseUnit>().HP -= damage;
    Destroy (gameObject);
}

public interface IBaseUnit
{
    int HP { get; set; }
}

public class Solder: MonoBehaviour, IBaseUnit
{
    public int HP { get; set; }

    //остальной код класса...
}

void OnTriggerEnter2D (Collider2D other)
{
    other.gameObject.GetComponent<IBaseUnit>().HP -= damage;
    Destroy (gameObject);
}