Izzet Prowess is a red-based deck with a small blue component (Preordain and Expressive Iteration are its only blue spells), so Mystical Dispute only costs {U} against those cards. Against Lightning Bolt, Lava Dart, Cori-Steel Cutter and their red creatures it costs the full {2}{U}, which still makes it a flexible counter for their key spells. A second Spell Snare picks off their two-mana spells like Expressive Iteration, Cori-Steel Cutter, or Slickshot Show-Off, and Surgical Extraction can strip Mishra's Bauble or a key burn spell to slow their aggression. Silence doesn't stop their hasty creatures or already-resolved threats and is too narrow against their low curve, while Day's Undoing is risky since it can refill their hand with gas against your removal-light plan post-sweep. Keep your life-gain and removal suite (Solitude, Wrath of the Skies, Supreme Verdict) to survive their burn-heavy aggression.