Mulligans
General Mulligans
Get Lady In White as quickly as possible to increase the value of your minions. Shadow Essence puts your minions into the dead pool. Vivid Nightmare and Mind Blast are for the kill combo, so it's good to have them.
Aggro Mulligans
Get taunts out as quickly as possible and bring them back continuously. Witchwood Grizzly is excellent against aggro, so Divine Spirit on that effectively heals for ~12 health. Use Shadow Visions to grab any of these spells, or Inner Fire to combo with a doubled-health Grizzly.
Control Mulligans
Against Control, you don't want grizzly at all since against most control decks it will be a 3/5 at best on turn 5. Get a minion out with Shadow Essence and bring it back and combo off of it with Divine Spirit.
Use the effect of Lady In White to make your minions much stronger. Grizzly, Sleepy Dragon, and Ysera have 12 attack, Hungry Ettin becomes a 6 mana 10/10 and Primordial Drake and Obsidian Statue become 8/8’s. Velen can be combo’d with Vivid Nightmare, Malygos, and Mind Blast. By turn 10 you’ll have reduced your opponent to 20 health if they haven’t conceded already from having to deal with massive taunts.