The strategy of the deck is to use Sporefolk to bin massive amounts of power so you can kill with Stonescar Excavator in 1-2 turns. You almost always want to mulligan since most hands are going to have 4-6 power. 4 power hands are fine to keep however if it contains a Sporefolk. Once you get a Sporefolk the goal is to bin as much as possible with Dark Returns and Mirror Images. Then the goal is to obtain Stonescar Excavator out either by drawing it or through Whispering Wind. Because of this it is can be important to save a Dark Return in case a Whispering Wind gets binned. Quarry and Herald's Song act as more ways to bin power as well as dig for the important cards.
As for sideboard I'm not really sure. I think Lightning Storm would be important to deal with Aggro, although it does kill your units. Plague and Stray would also be viable options. Ghostform might be helpful to keep activating Whispering Winds. 1 of Groundbreaker may also be good to deal with any Lifeforce decks.
V1.1: +1 Whispering Wind, -1 Herald's Song.