Missing sideboard cards? The full 15-card sideboard runs about $44.49.
Buy sideboard on ManapoolStandstill is actively bad here since Stiflenought wants to deploy a single cheap threat (Phyrexian Dreadnought) and protect it, meaning breaking Standstill often just draws them three cards after they combo off. Meddling Mage is the best card in the matchup, naming Phyrexian Dreadnought, Stifle, or Counterspell to strip away their plan entirely. Annul answers Phyrexian Dreadnought itself or Powder Keg before it resolves. Wrath of God is often too slow versus a turn-one 12/12, so trim to one copy's worth of removal-heavy plan and lean on Swords to Plowshares plus counter magic to control the pace.
Mono-Red Sligh is a fast, low-toughness aggro deck where Wrath of God is often too slow and Humility does little against a board of 1/1s and 2/1s that just race you anyway. Circle of Protection: Red is excellent insurance against their reach (Fireblast, Lightning Bolt, Incinerate) and buys crucial time. Teferi's Response protects Mishra's Factory and other lands from Wasteland while cantripping, and the extra Disenchant answers Cursed Scroll or Seal of Fire. Prioritize Swords to Plowshares and Standstill to stay ahead on tempo and card advantage.
Enchantress has almost no creatures to Wrath away, so sweepers are largely dead weight; Powder Keg is also awkward since most of their permanents (Serra's Sanctum, Sterling Grove, enchantments) survive at low fuse counts or are protected. Exalted Angel is a fast, evasive clock that dodges their removal-light plan and gains life against Parallax Wave beats. Annul is a proactive answer to Sterling Grove, Solitary Confinement, or Opalescence before they resolve, while the extra Disenchant helps break through Solitary Confinement or kill Serra's Sanctum's enabler enchantments. Keep Counterspell and Standstill as your main way to disrupt their engine before it snowballs.