Mono Red Madness wants to dump burn into our face, discarding into Fiery Temper madness while Kessig Flamebreather and Guttersnipe add extra damage for every spell they cast, so cheap interaction that stops a key spell is premium. Spellstutter Sprite comes in as a flash flying blocker that can counter a spell with mana value equal to or less than the number of Faeries we control. Alone it counts only itself, so it answers one-mana plays like Lightning Bolt, Lava Dart, Faithless Looting, or Voldaren Epicure. With a Sneaky Snacker (also a Faerie) on the battlefield it can also hit two-mana spells like Kessig Flamebreather or Grab the Prize. It can't stop Fiery Temper or Guttersnipe without more Faeries, so aim it at the cheap spells that fuel their chain. Writhing Chrysalis is a slow 4-mana body that does little against their fast clock, so it comes out. Prioritize racing with lifelink (Sacred Cat, Outlaw Medic) and using Prismatic Strands to blank a lethal burn turn.

Dimir Control's removal (Snuff Out, Cast Down, Agony Warp) and counterspells (Counterspell, Spell Pierce) are a grindy gauntlet, and most of our sideboard does little against them. Red Elemental Blast can hit their blue spells and permanents (Counterspell, Spell Pierce, Agony Warp, Lorien Revealed, and their blue creatures), but it can't touch black cards like Snuff Out and Cast Down, so it is too narrow to justify cutting from our core. Spellstutter Sprite only counters a spell with mana value no greater than the number of Faeries we control, so a lone Sprite can't answer Counterspell or Lorien Revealed, and we'd rather keep our full complement of resilient value engines (Malevolent Rumble, Pursue the Past) against a grindy matchup. Staying the course and leaning on card advantage via Pursue the Past and Bitter Reunion to out-grind their removal-heavy shell is the better plan than thinning our proven core. Keep attacking with efficient threats they must answer one at a time.

Affinity's board is full of nonland artifact creatures and artifact lands, and Ancient Grudge destroys any of their key nonland artifacts (Refurbished Familiar, Utrom Monitor, Baleful Strix, Cryogen Relic) with flashback for a two-for-one; note it cannot hit their indestructible artifact-land Bridges (Razortide Bridge, Goldmire Bridge, Mistvault Bridge). Bitter Reunion is a slower card-filtering piece that doesn't interact with their board, so it's the easiest cut. Keep Thraben Charm in reserve as it already hits for 2x creature count against their small creatures. Focus removal on Refurbished Familiar and Utrom Monitor since those are the biggest bodies that dodge combat easily.