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Seeing as this deck only utilizes a Stage one and a Basic Pokemon, you can keep the Pokemon in your deck down to six cards

Llanowar Elves is worlds better since turn-1 ramp can be a killer, but that extra mana for a bit more resiliency does help against certain decks

Other must kill creatures provide multiple targets for removal, making it harder for an opponent to ensure they can take care of Niv-Mizzet

It doesn't copy whether the creature is tapped or untapped, whether it has any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on

Crackling Drake serves as another win condition and a great speed bump on the way to Niv