Missing sideboard cards? The full 15-card sideboard runs about $9.91.
Buy sideboard on ManapoolGolgari Midrange is grindy with efficient removal (Fatal Push, Abrupt Decay, Go for the Throat) and big late threats like Sheoldred, the Apocalypse and the Unholy Annex Demon, so combat tricks that invite two-for-ones lose value. Redcap Melee deals 4 damage, enough to kill Sentinel of the Nameless City, Graveyard Trespasser or Badgermole Cub, but it will not kill the 4/5 Sheoldred. Every creature they play is nonred, so each cast also costs you a land. Use it sparingly and only when the tempo is worth it. Fiery Impulse handles Badgermole Cub, and with two or more instants and/or sorceries in your graveyard it deals 3 and can also kill Graveyard Trespasser. Abandon Attachments lets you discard a spare card, such as an extra land, to draw two against their Thoughtseize-heavy disruption. Keep the rest of the aggressive burn and prowess core intact since racing before Sheoldred's life-loss trigger snowballs is still the winning plan.
This mirror-ish matchup comes down to who protects their threats and answers the opponent's big finishers, Hearth Elemental and Eddymurk Crab. Spider-Sense lets us counter their Prismari Charm, Get Out, or Fiery Impulse on the cheap via Web-slinging (which means returning a tapped creature of ours), and it also hits the sorcery side of Hearth Elemental. That matters because Stubborn Denial already contests our noncreature spells. Their creatures are large (Hearth Elemental is a 4/5, Eddymurk Crab and Sunderflock are 5/5s), so our small burn spells mostly can't kill them. A kicked Firebending Lesson, for 5 mana total, deals 5 damage and is our clean answer to those threats. Octopus Form gives a prowess creature hexproof, so it can dodge Fiery Impulse, Prismari Charm's damage or bounce mode, and Fading Hope. Monstrous Rage is cut because a small damage boost matters less against their big blockers. Reckless Rage is cut because its 4 damage doesn't kill their creatures and it deals 2 damage to one of our own. One of the two maindeck Boomerang Basics is trimmed to make room. Prioritize racing with prowess creatures while holding up interaction to protect your board from their removal suite.
UBR grinds with Fable of the Mirror-Breaker, Indomitable Creativity, and a cheap burst of burn (Fiery Impulse, Volcanic Spite), with top-end bombs like Atraxa, Grand Unifier (white-blue-black-green) and Valgavoth, Terror Eater (black), so targeted disruption beats raw combat tricks. Precise Redaction only hits white or black spells, which here means just Atraxa and Valgavoth, and their removal and sweepers are red, so it is too narrow and stays in the sideboard. Rending Volley deals only 4 damage, which cannot kill a 7/7 Atraxa, and it cannot target Valgavoth, so it stays out as well. Instead, Spider-Sense counters their instants and sorceries (the burn, Negate, Make Disappear, and Indomitable Creativity), and Aether Gust answers red or green spells and permanents such as Fable, Indomitable Creativity, their burn, and Raph & Mikey. Monstrous Rage is weak here since their cheap removal (Fiery Impulse) punishes committing resources to a creature, so trim it for interaction.