Let me gather the data needed to answer this. I'll look up Mono Green Landfall in Pioneer and identify its worst matchups, then check sideboard cards.
Good, it's a Standard archetype. Now let me get the meta report to find the top archetypes, and pull the sideboard cards for Mono Green Landfall simultaneously.
Now let me check Mono Green Landfall's head-to-head records against the most prominent archetypes to find the worst matchups.
The clear worst matchup is Izzet Spellementals at 38.5%. Let me also check Dimir Midrange and Lifegain, then look at which sideboard cards the deck plays more frequently in weeks where it likely faces Spellementals, plus a sources pull.
Here is the full breakdown for @Zacktrack92:
Matchup summary (last 60 days, Standard):
Mono Green Landfall sits at a solid 55% overall winrate, but its clearest problem matchup is Izzet Spellementals at only 38.5% (42W-67L across 109 matches). Izzet Prowess is roughly even at 52.3%, while Selesnya Ouroboroid is also close to parity at 51%. Everything else, including Jeskai Control (56.4%), Jeskai Lessons (58.4%), Dimir Excruciator (59.5%), and Dimir Midrange (62%), leans in Landfall's favor, so Spellementals is the real nemesis here.
Sideboard cards most likely helping against Spellementals and the other bad matchups:
Against Izzet Spellementals (the main threat), the deck's sideboard pattern points to a few high-presence cards. Torpor Orb appears in 61% of sideboards at 2.3 copies on average, which directly suppresses enters-the-battlefield triggers that Spellementals abuses. Surrak, Elusive Hunter (84% presence) provides a hexproof threat that dodges the tempo-based interaction those decks rely on. Meltstrider's Resolve (95% presence, nearly universal) looks like the primary protection/resilience piece. Warg Tactics (34%) and Snakeskin Veil (20%) round out the protection suite for fighting through removal-heavy blue-red piles. Soul-guide Lantern (56%) handles any graveyard synergies. Against Selesnya Ouroboroid near parity, Keen-Eyed Curator (76%) and Mossborn Hydra (87% at 3.6 copies) look like the go-wide threats to overpower a stalled board.
In short: Torpor Orb and Surrak, Elusive Hunter are your most targeted hate pieces for the Izzet Spellementals matchup specifically, while Meltstrider's Resolve is the universal glue almost every build runs.
Sources: Data from 75 tournaments over the last 60 days, 80% MTGO and 20% Melee. Recent examples:
https://melee.gg/Decklist/View/c03ebfbc-9056-4696-872d-b4bd012250f2
https://melee.gg/Decklist/View/ddeda495-be44-4f2d-b1ad-b4bd00e037d8