I went from 39.5 million attack to 108 million attack by buying 100 each of the highest unit in each area (ground, air, etc) and 50 each of the highest valor unit. I still have 10 billion in cash to buy more or to replace losses during the upcoming WD event. With the valor units being indestructible, I think I'm set up pretty well...Just need to update my hospital since I've never had to worry about that before.