Izzet Spellementals is almost entirely red and blue spells feeding Hearth Elemental and Eddymurk Crab. Flashfreeze is a clean answer to their red spells, including Hearth Elemental, Burst Lightning, Impractical Joke and the red-blue gold spells like Prismari Charm and Traumatic Critique. It can't touch mono-blue cards such as Eddymurk Crab or Sunderflock. Negate backs this up against Prismari Charm, Traumatic Critique and their other noncreature spells. Requiting Hex stays in the sideboard because it only destroys creatures with mana value 2 or less, and their creatures (Hearth Elemental, Eddymurk Crab, Sunderflock) all cost far more, so it has no real targets. We trim the slower grindy pieces (Rapid Rescue, Awaken the Honored Dead) since this matchup rewards proactive disruption over incremental card advantage against a deck that outgrinds through volume of cheap spells.

UR Aggro wants to chain cheap cantrips and burn into evasive threats like Slickshot Show-Off, so proactive interaction that stops their spells or kills cheap creatures is better than our slower value plays. Flashfreeze counters only red or green spells, so it won't touch their blue cards, but it is a clean hard counter for their red ones: Slickshot Show-Off, Burst Lightning, Impractical Joke, Vibrant Outburst, and Wild Ride. Negate stops their noncreature spells such as Flow State, Burst Lightning, Impractical Joke, or Stock Up, but it can't hit their creatures. Requiting Hex is clean removal for Elusive Otter or Slickshot Show-Off, since both are mana value 2 or less. If we control a creature, we can also blight 1 for the additional cost and gain 2 life, which helps in the race. Cut the slower, less impactful Rapid Rescue and Wistfulness copies plus Awaken the Honored Dead since we need to be faster and more reactive rather than grinding.

Dimir Midrange relies on noncreature pieces like Requiting Hex and Kaito, Bane of Nightmares, so Duress proactively strips these (along with their other removal and counterspells) before they generate advantage. Requiting Hex answers their own cheap threats such as Dream Beavers, Floodpits Drowner, or Azure Beastbinder at mana value 2 or less. We trim the slower mill and card-selection cards since this mirror-adjacent matchup rewards disruption and efficient removal over incremental card selection. Bringer of the Last Gift and our bigger threats stay in as they outsize their midrange creature suite.