Mainboard (60)
Creatures (10)
Instants (29)
Sorceries (5)
Lands (16)
Mono-Red Madness wins by flooding the board with cheap threats like Guttersnipe, Kessig Flamebreather, and Sneaky Snacker while generating card advantage through Faithless Looting and Highway Robbery. Blue Elemental Blast is your best sideboard card here, countering or destroying their key threats and spells. Snuff Out and Cast Down handle their creatures efficiently, and Tolarian Terror and Gurmag Angler are large enough to survive most of their burn. Cut the situational Spell Pierce (too conditional against a mostly permanent-based deck), Deep Analysis (too slow at sorcery speed), Murmuring Mystic (5 mana is too slow), and the maindeck Unexpected Fangs since you're bringing in the additional copies already in the sideboard.
Grixis Affinity is a fast artifact-based synergy deck that wins through artifact count payoffs like Myr Enforcer and Utrom Monitor backed by Galvanic Blast for removal. Annul is your best card in this matchup, countering key artifacts like Ichor Wellspring, Thoughtcast, and the threats themselves at the cost of a single blue mana. Abandon Attachments comes out entirely since the opponent has very few enchantments and sorcery-speed removal is poorly positioned against their fast clock. Murmuring Mystic is too slow and provides tokens that don't race well against artifact creatures, while the extra Unexpected Fangs help your Tolarian Terrors and Sneaky Snackers trade favorably or survive Galvanic Blast. The primary game plan is to counter their key artifacts early and close quickly with your delve threats before Krark-Clan Shaman can machine-gun your board.
The mirror match is decided by who resolves a threat first and protects it through the opponent's counterspell suite. Nihil Spellbomb is your most important sideboard card, as both Thought Scour and Mental Note fuel graveyard-based threats like Tolarian Terror and Gurmag Angler — exiling the opponent's yard at the right moment can strand multiple would-be threats. Murmuring Mystic is too slow and fragile in a matchup defined by Snuff Out and Cast Down, and Spell Pierce loses significant value in a grindy counter war where both players will have mana to pay. Prioritize holding up Counterspell and Snuff Out to answer the first threat, and play your own threats into windows where the opponent is tapped out or committed resources to a spell.