I like the idea of playing the Nivmagus Elemental over enchantments, because of enchantments are being played at sorcery speed. I could play instants that give more power and toughness buffs, but they lack scry. Perhaps he is best left to the side board, since I am imagining he will be most useful against control decks responding to the instants that I am attempting to cast, and I respond by exiling it and buffing the Elemental. I do like Phalaxan 2 drop, and may consider putting him in for the Nivmagus, but side board the Nivmagus in for the control match ups.
You may think I am crazy in regard to Spark Jolt, but I didn't plan to use it for removal. I would be using it to target Fabled Hero, Anax, or Favored Hoplite to trigger Heroic, plus scry 1. Magma Jet hits too hard and costs too much to be able to cast a bunch of instants by turn 4, where I would preferably have the Fabled Hero on the battlefield.