Mulligans
General Mulligans
Your early game ramp is super important, and Oaken Summons is great against all kinds of decks.
Aggro Mulligans
Against Aggro, your early game removal is more important than anything else. Greedy Sprite is also a great pick since it ensures Oaken Summons brings out your Golems. Golem on it's own also isn't too bad, especially if you have no other early tools.
Control Mulligans
Against Control decks, the Twig is your best pick. Wild Growth is also solid.
This deck is pretty fun right now because it has a decent chance of beating two really popular decks: Baku Face Hunter and Shudder Shaman.
Against aggro, Druid’s incredible armor gain and control tools allow you to stall your opponent out of threats. Oftentimes, you won’t even need the combo (or the full combo) in order to outlast them. Against Baku Hunter, Malfurion the Pestilent is the MVP.
Against control, you want to ramp, cycle, and control the board. Getting the combo pieces together can be tough, but eventually you can kill them in one turn with spell damage. Shudder Shaman can heal a lot, but you can stall their OTKs slightly by gaining tons of armor.
Your idea combo is Ixlid, Fungal Lord + Faceless Manipulator + Twig of the World Tree‘s deathrattle + Malygos, giving you four spell damage dragons and making each Moonfire deal 21 damage. Of course, the full combo might not assemble itself, so there are other combinations as well. Against aggro, putting Malygos on an empty board can often be enough.
This deck only runs one card from The Witchwood: Countess Ashmore. She helps you tutor Twig of the World Tree fairly reliably; by turn 7 you generally have played your Greedy Sprite either from the hand or with Oaken Summons.