I deleted my progress but new boards cost what they did before I deleted, and Extreme Gardening and PoopMongering say that i have 0 of those boards left, I only have the first board though
Build up your army first, that should be obvious. Then just focus on taking the battlefield little by little, trust me, it works. I beat the last 7 or 8 levels without using my hero besides for spells. Once you get this down, its quite simple really.