Mulligans
General Mulligans
This is a late-game deck, so you need to mulligan for the few semi-early game cards in it, in Firefly, Tar Creeper, and Hot Spring Guardian. The other cards are for the combo, which it would be good to have both Spellstone and Earth Elemental in your hand at least by turn 5. Grumble is vital to the combo, but you don't need to draw him until turn 10.
This deck uses Elementals, but alongside that a combo with Sapphire Spellstone and Grumble. On turn 5 you play Earth Elemental, turn 6 play Tar Creeper or something, turn 7 Blazecaller, Turn 8 Hallazeal. On turn 9 play a second Earth Elemental, and this whole time have the Spellstone in hand (or maybe both copies). Turn 10 you have 7 mana, so play Grumble and then the now 1-mana Blazecaller or Kalimos. Turn 11 you can play both Earth Elementals and Spellstone it, so now you have 5 7/8 taunts, a 7/7, and a 6/6 or a 7/7. Concede?
As a backup you can play Hallazeal into Volcano, then Arcane Tyrant. Heal for 15, then play a 4/4 so you have a body on the board. Volatile Elemental could be taken out for Prince Keleseth.