“Affinity is a white-blue-black artifact aggro deck with no red cards. Its threats are artifact creatures made cheap by affinity: Myr Enforcer (4/4), Refurbished Familiar (2/1 flier), Utrom Monitor (3/3 flier) and Baleful Strix, alongside the noncreature artifacts Cryogen Relic, Giant's Boulder, Blood Fountain and Nihil Spellbomb, so nearly every permanent it casts is an artifact spell. Glint Hawk is a white Bird creature, not an artifact: it returns one of its artifacts to hand when it enters. Its other spells are Dispatch and Reckoner's Bargain (both instants) and Thoughtcast (a sorcery). Its blue cards are Utrom Monitor, Cryogen Relic, Baleful Strix and Thoughtcast.”
Missing sideboard cards? The full 15-card sideboard runs about $93.67.
Buy sideboard on ManapoolMono Red Madness wants to dump its hand fast and chain madness triggers off Faithless Looting and Lava Dart, so Arms of Hadar is a great sweeper here, giving every creature in the deck -2/-2 and wiping Voldaren Epicure, Sneaky Snacker, and Guttersnipe (though not Kessig Flamebreather at 1/3 toughness). Envelop counters key sorceries like Faithless Looting or Grab the Prize, cutting off their card velocity. Reckoner's Bargain is too slow and clunky against a deck racing to kill us quickly, and one Blood Fountain is excess value we can't afford to prioritize over interaction. Hydroblast and Pyroblast stay in the board since the opponent's lone blue card, Sneaky Snacker, never gets cast.
Duress strips Counterspell, Snuff Out, or Dispel from their hand before we commit our threats, which is excellent proactive disruption against a grindy control deck. Tithing Blade forces an immediate sacrifice, which is awkward value against a deck that leans on single copies like Thorn of the Black Rose or Mukotai Ambusher. Reckoner's Bargain is too slow in a matchup decided by card advantage and tempo, not life gain, and the second Nihil Spellbomb is redundant graveyard hate against a deck with minimal graveyard synergies. Hydroblast and Blue Elemental Blast stay boarded since this opponent's maindeck plays no red spells for them to hit; Pyroblast-style hate is not in our sideboard anyway.
Spy Combo relies on Dread Return's flashback (sacrifice three creatures) and Balustrade Spy triggers to mill and reanimate, so exiling their graveyard with Nihil Spellbomb directly disrupts both the Dread Return flashback plan and anything they want back. Duress strips their Dread Return or a key enabler before they can set up. Blood Fountain and Reckoner's Bargain are both too slow and value-oriented against a combo deck that wants to go over the top of a fair aggro plan, so our best path is to be faster and more disruptive rather than greedier.