Oh, a few pointers:
After you buy 200 of a hero, they start getting DPS multipliers every 25 levels. Normally 4x, but you get a bonus of 10x at levels 1k, 2k, and 3k. So it's worth it to go back and buy more low level heroes once you reach a certain point. The bonus stops at 4100, so don't bother after that. Once you get to the point you start focusing on maxing them out, only use Treebeast, Ivan, Brittany, Samurai, Forest Seer, and Frostleaf. Because of the 4100 cap, you'll just get to the point where you buy enough of a hero to get their bonuses, and then only buy Frostleaf.
Best advice to give, is that when the game starts to feel like it's progressing slower than you want, ascend.