Missing sideboard cards? The full 15-card sideboard runs about $7.97.
Buy sideboard on ManapoolWinternight Stories is slow against a deck full of Thoughtseize and Duress, and Get Out is the weakest of your interaction here, so both come out. Bring in Abrade to kill Graveyard Trespasser, Badgermole Cub, and an animated Mutavault. Its 3 damage won't kill Sheoldred, the Apocalypse or the 6/6 Demon token from Unholy Annex/Ritual Chamber, so don't count on it for those. Negate counters noncreature spells such as Culling Ritual, Professor Dellian Fel, and Unholy Annex/Ritual Chamber. It can't stop a planeswalker's loyalty abilities once Dellian Fel is on the battlefield. Ashiok, Dream Render's -1 exiles their graveyard, which blanks graveyard plans like Takenuma, Abandoned Mire's channel ability. Keep Stubborn Denial and Prismari Charm in as your main interaction. Stubborn Denial counters noncreature spells like Thoughtseize, Fatal Push, Culling Ritual, and Professor Dellian Fel, but it can't hit creature spells. Prismari Charm can bounce Sheoldred, the Apocalypse or Badgermole Cub. Watch out for Thoughtseize and Duress stripping your interaction before you can deploy it.
Selesnya Company floods the board with small creatures (Elvish Mystic, Llanowar Elves, Badgermole Cub, Collected Company hits), so Anger of the Gods is a huge blowout that exiles most of their mana dorks and 2-3 toughness threats before they can go wide. Abrade cleanly kills Skyclave Apparition, Archon of Emeria, or any dork, and can also hit Mutavault-style lands if needed elsewhere, but here mainly answers their small creatures. Traumatic Critique and Winternight Stories are too slow and card-disadvantageous against their proactive curve-out plan. Keep Stubborn Denial and Fiery Impulse as efficient interaction, noting Archon of Emeria restricts you to one spell per turn so sequence carefully.
Mono-Red Prowess is extremely aggressive with a low curve of small creatures (Monastery Swiftspear, Soul-Scar Mage, Emberheart Challenger), making Anger of the Gods a strong sweeper that deals 3 damage to each creature and exiles most of their early plays instead of letting them die. It also hits your own creatures, but Hearth Elemental, Eddymurk Crab and Sunderflock all survive it. Be careful with Screaming Nemesis: it triggers whenever it is dealt damage, so Anger's 3 damage still makes it deal 3 damage to another target of the opponent's choice, even though it gets exiled afterward. Abrade's 3-damage mode can kill Screaming Nemesis (3 toughness) but triggers the same redirect, and it can chip Sunspine Lynx (4 toughness, so it needs help). It can also kill Mutavault, but only while Mutavault is animated as a creature. Mutavault is a land, so Abrade's destroy-artifact mode has no targets in this matchup. Traumatic Critique and Winternight Stories are far too slow and card-disadvantageous in a race against their hyper-aggressive curve. Lean on Fiery Impulse and Stubborn Denial for cheap interaction, and note Sunspine Lynx prevents you from gaining life if that becomes relevant.