Spell Snare is narrow here: it only hits mana value 2 spells (Basking Broodscale, Malevolent Rumble, Talisman of Resilience) and misses their real threats like Emrakul and Devourer of Destiny. Harbinger of the Seas turns off Eldrazi Temple, Urza's Saga, and Ugin's Labyrinth, crippling their mana. Nihil Spellbomb keeps their graveyard empty, which holds down Emrakul's cost reduction and Shifting Woodland's delirium, while Engineered Explosives (set to the right mana value) sweeps Blade of the Bloodchief and other cheap artifacts like Springleaf Drum and Vexing Bauble. Dress Down strips Emrakul's protection from instants and shuts off Basking Broodscale's adapt for a turn. Keep Thoughtseize to strip Emrakul or Devourer of Destiny from their hand, since Fatal Push can't kill Devourer (mana value 7), and keep Fatal Push to kill Basking Broodscale before it scales. Sheoldred's Edict is weaker here since they choose which nontoken creature to sacrifice and can give up a cheap Basking Broodscale instead of a real threat.

The combo lives on Urza's Saga assembling Blade of the Bloodchief and Eldrazi Temple/Ugin's Labyrinth ramping into the backup Devourer of Destiny/Emrakul plan, so Harbinger of the Seas turning all their nonbasics into Islands is devastating to both halves of their gameplan (it turns your own nonbasics into Islands too, so weigh that against your black mana). Engineered Explosives is a cheap, flexible answer: set to 1 counter it destroys Blade of the Bloodchief, and set to 0 it wipes out the Urza's Saga Construct and Eldrazi Spawn tokens. Nihil Spellbomb doesn't destroy anything; it exiles their graveyard, which blunts Emrakul's cost reduction (it counts card types in its caster's graveyard) and Shifting Woodland's delirium. Spell Snare is narrow here: it only counters mana value 2 spells, so it catches Basking Broodscale and Malevolent Rumble but misses Blade of the Bloodchief (1), Glaring Fleshraker (3), Devourer of Destiny and Emrakul, which makes it the first thing to shave. Prioritize Thoughtseize and Fatal Push to strip or kill Basking Broodscale and Glaring Fleshraker before the loop starts (Push only reaches Fleshraker with revolt, such as after cracking a fetchland), and hold Counterspell for the combo pieces themselves, like Basking Broodscale and Glaring Fleshraker. Urza's Saga is a land and its activations are abilities, so Counterspell can't touch them.

Stern Scolding cleanly answers Dragon's Rage Channeler, Monastery Swiftspear, and Slickshot Show-Off for one mana. Consign to Memory counters triggered abilities such as prowess, Cori-Steel Cutter's flurry, and Channeler's surveil, and it also hits colorless spells like Mishra's Bauble. Replicate lets you copy it to cover more than one trigger on turns they chain spells. Spell Snare stays in because it counters mana value 2 spells, which covers Slickshot Show-Off, Cori-Steel Cutter, and Expressive Iteration. It can't hit their one-mana cards such as Lava Dart, Lightning Bolt, or Mutagenic Growth. Force of Negation also stays in, since most of their deck is noncreature spells (Lightning Bolt, Lava Dart, Mutagenic Growth, Cori-Steel Cutter, Expressive Iteration, Preordain) and it can be cast for free on their turn. To make room, cut the slow cards that do little against a fast, cheap-spell deck: the five-mana Quantum Riddlers, the Tamiyos, the Flow State, and the Sheoldred's Edict. Edict is weak against many small creatures and Monk tokens. Keep Counterspell and Thoughtseize as your best interaction, and prioritize Orcish Bowmasters and Psychic Frog to stabilize the board against their low-toughness creatures.