“Living End is a cascade combo deck: it cycles creatures into its own graveyard, then casts Shardless Agent or Violent Outburst to cascade into Living End (mana value 0 and no mana cost, so it is cast through cascade; suspending it is a slow backup), which returns every cycled creature at once. Because cascade hits the first nonland card with mana value below 3, Living End must be the only such card in the deck: any sideboard card with mana value 0-2 (for example Inevitable Betrayal) may only come in by replacing Living End copies one-for-one, and Inevitable Betrayal is only worth that swap against decks whose best creatures are worth stealing (Eldrazi, big ramp threats), never against creature-light combo decks. The deck runs only about a dozen true lands, so Oliphaunt and Generous Ent are part of the mana base via landcycling as well as graveyard fodder, and Street Wraith is free cycling; none of these are "clunky top-end". Flexible slots to trim for sideboard cards are Force of Negation against creature decks (it only counters noncreature spells), Subtlety, Endurance, Wistfulness, Sink into Stupor, Colossal Skyturtle, Curator of Mysteries and one or two Waker of Waves; Mystical Dispute, Force of Vigor, Clarion Conqueror, Teferi, Time Raveler and Brotherhood's End all cost 3+ and are cascade-safe.”
Missing sideboard cards? The full 15-card sideboard runs about $57.75.
Buy sideboard on ManapoolMono-Green Eldrazi leans heavily on artifacts and lands like Eldrazi Temple and Urza's Saga, plus big colorless threats. Force of Vigor blows up Urza's Saga before it assembles, and Brotherhood's End sweeps their small Eldrazi Spawn tokens and Basking Broodscale-style creatures at 3 toughness or less. Clarion Conqueror is a 3/3 flyer that shuts off activated abilities of artifacts, creatures, and planeswalkers, which turns off Springleaf Drum, Talisman of Resilience, Blade of the Bloodchief's equip, Basking Broodscale's adapt, and the mana ability on Eldrazi Spawn tokens. It does not stop Urza's Saga, which is an Enchantment Land, so its Construct-making chapter still works and Force of Vigor is the answer there. Force of Negation only hits noncreature spells. It can answer Ancient Stirrings, Malevolent Rumble, or Kozilek's Command, but the opponent's real threats are creature spells it can't counter, so it comes out along with clunkier top-end and a redundant card-selection piece.
This Eldrazi combo deck wins through Blade of the Bloodchief equipped to Basking Broodscale, so Force of Vigor is a premium answer, destroying Blade (and incidentally Vexing Bauble or Urza's Saga) at instant speed, even for free on their turn by exiling a green card from our hand if we're tapped out. Brotherhood's End destroys all artifacts with mana value 3 or less, sweeping Blade of the Bloodchief and Vexing Bauble, or its first mode kills their 2/2 Basking Broodscale and Eldrazi Spawn tokens outright. Both Force of Vigor (mana value 4) and Brotherhood's End (mana value 3) are cascade-safe since Shardless Agent and Violent Outburst only hit nonland cards with mana value below 3. Force of Negation is cut because most of what we need to stop is a creature or an equip activation, neither of which it can target, and Sink into Stupor is too slow against a deck that can go off suddenly.
Izzet Prowess is a blue-red tempo deck full of cheap noncreature spells, so Mystical Dispute is excellent, countering their burn and card selection for as little as {U}. Teferi, Time Raveler shuts off their instant-speed interaction on our turn and bounces a problematic Dragon's Rage Channeler or Slickshot Show-Off while drawing a card. Endurance is far less valuable here since the opponent has no real graveyard plan worth attacking, and Sink into Stupor is replaced by more proactive countermagic. Keep Force of Negation in, since it still catches their key noncreature spells like Expressive Iteration or a late Cori-Steel Cutter.