Mulligans
General Mulligans
Generally, you want to keep a Dragon, most likely an Emerald Explorer in case of Breath of Dreams, Overgrowth is always a keep, unless you have 2 copies of it. Nature Studies is versatile too and can lead to a turn 3 Overgrowth
Aggro Mulligans
Aggro you want some sort of Taunt and Ramp
Control Mulligans
Control is slower so an option is to Shan'do your Winged Guardian, especially after one or two Survival of the Fittest.
Honestly Ramp it the best thing in this deck. An early Winged Guardian shuts down most agro if you don’t draw Guardian Animals. Survival of the Fittest is great whenever you have a board, I.E. after Ysera. Can we talk about how absurd it is that Breath of Dreams and Overgrowth are in the same rotation? Like Jez, my Opponent sometimes has 4 Mana and here I am at 7 or 8.