Reconsider Madcap Skills. It leaves you open to be 2-for-1'd with a cheap burn or removal spell.

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Use Chandra's Phoenix until we get Purphoroses. Young Pyromancer and Molten Birth are probably not the best choices without Purphoros.

Skullcrack might be better than Massive Raid. Same cost, Skullcrack always deals 3 damage but Massive Raid depends on having enough creatures. Massive Raid could work well after Molten Birth, however.

  3 Purphoros
  2 Young Pyromancer
  3 Molten Birth
  3 Massive Raid

  or

  3 Chandra's Phoenix
  2 Guttersnipe
  3 Shock
  3 Rubblebelt Maaka or Skullcrack or Armed/Dangerous?

Armed/Dangerous is at best +4 for 2 mana. Rubblebelt Maaka is +3 for 1 and is uncounterable (it's an ability, not a spell).

Sideboard:

  3 Burning Earth
  3 Electrickery
  3 Mizzium Mortars
  3 Peak Eruption (if red becomes common in standard)
  3 Skullcrack