No, defeating the green faction is absolutely possible. In my playthrough (I played on "warrior" difficulty), I did it. There were 200 dragons. The battles in the fourth mission were difficult, but manageable. Against 30 titans, I used 40 archdevils, although the scale wasn't quite the same. In any case, I think "Divine Retribution" will help you: they killed your titans, so you "avenged" the horses. Although, Ser9K, you clearly overdid it with the troops.