As promised, I made some calcs of Geomancy Sylveon and determined the mons that could stop its sweep after a boost (using HP Fire IVs):
If Sylveon has HP Fire, it's walled by Heatran and Chansey
If Sylveon has HP Ground, it's walled by M-Scizor, Ferrothorn, Chansey and Bronzong (using Shadow Ball can alleviate its matchup against certain mons like Heatran or Bronzong) and I've probably forgot some mons for one/both of the HPs
I didn't include Sash leads like Mamoswine, Scolipede and Azelf since they're used as dedicated leads and by the time Sylveon sets up, they'll either be already KOed or having their Sash broken (or won't be at full HP at least)
As we can see, there's only a couple of mons that can stop this monster and the right conditions must be met to do so or most of them will be KOed. Moreover, most of the listed mons are Megas or very passive, which means the number of Geomancy Sylveon checks will be very limited within a team and thusly makes it a very centralising (to not say broken) threat in the metagame.
252 Atk Tough Claws Mega Metagross Bullet Punch vs. 0 HP / 0 Def Sylveon: 270-318 (81.5 - 96%) -- 56.3% chance to OHKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 0 HP / 0 Def Sylveon: 606-714 (183 - 215.7%) -- guaranteed OHKO (Modest Sylveon can't even OHKO MegaGross at +2 with HP Fire/Ground)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 0 HP / 0 Def Sylveon: 606-714 (183 - 215.7%) -- guaranteed OHKO (Modest Sylveon can't even OHKO MegaGross at +2 with HP Fire/Ground)
252 Atk Excadrill Iron Head vs. 0 HP / 0 Def Sylveon: 384-452 (116 - 136.5%) -- guaranteed OHKO (Jolly Scarf Excadrill outspeeds and OHKOes Geomancy Sylveon, but if it's Timid instead of Modest, then...)
+2 252 SpA Sylveon Hidden Power Fire vs. 0 HP / 0 SpD Excadrill: 330-390 (91.4 - 108%) -- 50% chance to OHKO (68.8% chance to OHKO after SR)
252 Atk Landorus-T Earthquake vs. 0 HP / 0 Def Sylveon: 252-297 (76.1 - 89.7%) -- 18.8% chance to OHKO after Stealth Rock (Scarf Lando-T outspeeds and needs SR and some luck to OHKO Geomancy Sylveon, but if it's Timid instead of Modest, then...)
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Landorus-T: 408-480 (127.8 - 150.4%) -- guaranteed OHKO
44 Atk Technician Mega Scizor Bullet Punch vs. 0 HP / 0 Def Sylveon: 270-320 (81.5 - 96.6%) -- 62.5% chance to OHKO after Stealth Rock (if Bullet Punch doesn't KO, Sylveon needs HP Fire to OHKO M-Scizor back as Hyper Voice only has a slight chance to 2HKO it (guaranteed 2HKO after SR))
252+ Atk Choice Band Talonflame Brave Bird vs. 0 HP / 0 Def Sylveon: 334-394 (100.9 - 119%) -- guaranteed OHKO (doesn't work with any other Talonflame variant)
Thundurus can use Prankster T-Wave to let slower physical attackers KO Sylveon
+2 252 SpA Sylveon Hidden Power Fire vs. 0 HP / 0 SpD Excadrill: 330-390 (91.4 - 108%) -- 50% chance to OHKO (68.8% chance to OHKO after SR)
252 Atk Landorus-T Earthquake vs. 0 HP / 0 Def Sylveon: 252-297 (76.1 - 89.7%) -- 18.8% chance to OHKO after Stealth Rock (Scarf Lando-T outspeeds and needs SR and some luck to OHKO Geomancy Sylveon, but if it's Timid instead of Modest, then...)
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Landorus-T: 408-480 (127.8 - 150.4%) -- guaranteed OHKO
44 Atk Technician Mega Scizor Bullet Punch vs. 0 HP / 0 Def Sylveon: 270-320 (81.5 - 96.6%) -- 62.5% chance to OHKO after Stealth Rock (if Bullet Punch doesn't KO, Sylveon needs HP Fire to OHKO M-Scizor back as Hyper Voice only has a slight chance to 2HKO it (guaranteed 2HKO after SR))
252+ Atk Choice Band Talonflame Brave Bird vs. 0 HP / 0 Def Sylveon: 334-394 (100.9 - 119%) -- guaranteed OHKO (doesn't work with any other Talonflame variant)
Thundurus can use Prankster T-Wave to let slower physical attackers KO Sylveon
Mega Alakazam outspeeds Sylveon and can use Encore the turn after it used Geomancy to get free turns to switch in a physical attacker that could deal with Sylveon (but if it's Timid instead of Modest, then...)
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Mega Alakazam: 354-417 (141 - 166.1%) -- guaranteed OHKO
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Mega Alakazam: 354-417 (141 - 166.1%) -- guaranteed OHKO
252 Atk Jirachi Iron Head vs. 0 HP / 0 Def Sylveon: 312-368 (94.2 - 111.1%) -- 62.5% chance to OHKO (guaranteed OHKO after SR) (Scarf Jirachi outspeeds and can OHKO Geomancy Sylveon. Specially defensive variants also do a good job at tanking hits and 2HKOing it)
Klefki can use Prankster T-Wave to let slower physical attackers KO Sylveon
Klefki can use Prankster T-Wave to let slower physical attackers KO Sylveon
252+ Atk Life Orb Kabutops Waterfall vs. 0 HP / 0 Def Sylveon in Rain: 367-433 (110.8 - 130.8%) -- guaranteed OHKO (Kabutops needs rain to outspeed and OHKO Geomancy Sylveon)
80 Atk Life Orb Kingdra Waterfall vs. 0 HP / 0 Def Sylveon in Rain: 250-294 (75.5 - 88.8%) -- 12.5% chance to OHKO after Stealth Rock (again, Kingdra needs rain (and some luck) to outspeed and OHKO Geomancy Sylveon)
252 Atk Pure Power Mega Medicham Fake Out vs. 0 HP / 0 Def Sylveon: 104-123 (31.4 - 37.1%) -- 82.2% chance to 3HKO
252 Atk Pure Power Mega Medicham Bullet Punch vs. 0 HP / 0 Def Sylveon: 208-246 (62.8 - 74.3%) -- guaranteed 2HKO (after SR, Fake Out + Bullet Punch is a guaranteed KO)
252+ Atk Mega Swampert Waterfall vs. 0 HP / 0 Def Sylveon in Rain: 340-402 (102.7 - 121.4%) -- guaranteed OHKO (again, M-Swampert needs rain to outspeed and OHKO Geomancy Sylveon, but if it's Timid instead of Modest and M-Swampert loses the speed tie, then...)
+2 252 SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Mega Swampert: 468-552 (137.2 - 161.8%) -- guaranteed OHKO
252+ Atk Choice Band Victini V-create vs. 0 HP / 0 Def Sylveon: 573-675 (173.1 - 203.9%) -- guaranteed OHKO
32+ Atk Victini V-create vs. 0 HP / 0 Def Sylveon: 313-369 (94.5 - 111.4%) -- 68.8% chance to OHKO (guaranteed OHKO after SR) (as long as SR aren't up, Victini survives a boosted Hyper Voice and OHKOes Geomancy Sylveon back. Scarf variants can even outspeed Modest Sylveon at +2 and OHKO it with V-Create)
80 Atk Life Orb Kingdra Waterfall vs. 0 HP / 0 Def Sylveon in Rain: 250-294 (75.5 - 88.8%) -- 12.5% chance to OHKO after Stealth Rock (again, Kingdra needs rain (and some luck) to outspeed and OHKO Geomancy Sylveon)
252 Atk Pure Power Mega Medicham Fake Out vs. 0 HP / 0 Def Sylveon: 104-123 (31.4 - 37.1%) -- 82.2% chance to 3HKO
252 Atk Pure Power Mega Medicham Bullet Punch vs. 0 HP / 0 Def Sylveon: 208-246 (62.8 - 74.3%) -- guaranteed 2HKO (after SR, Fake Out + Bullet Punch is a guaranteed KO)
252+ Atk Mega Swampert Waterfall vs. 0 HP / 0 Def Sylveon in Rain: 340-402 (102.7 - 121.4%) -- guaranteed OHKO (again, M-Swampert needs rain to outspeed and OHKO Geomancy Sylveon, but if it's Timid instead of Modest and M-Swampert loses the speed tie, then...)
+2 252 SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 0 HP / 0 SpD Mega Swampert: 468-552 (137.2 - 161.8%) -- guaranteed OHKO
252+ Atk Choice Band Victini V-create vs. 0 HP / 0 Def Sylveon: 573-675 (173.1 - 203.9%) -- guaranteed OHKO
32+ Atk Victini V-create vs. 0 HP / 0 Def Sylveon: 313-369 (94.5 - 111.4%) -- 68.8% chance to OHKO (guaranteed OHKO after SR) (as long as SR aren't up, Victini survives a boosted Hyper Voice and OHKOes Geomancy Sylveon back. Scarf variants can even outspeed Modest Sylveon at +2 and OHKO it with V-Create)
Chansey isn't close of being 2HKOed by Geomancy Sylveon (even after SR!)
Empoleon can use Yawn to either make Sylveon asleep or force it to lose all of its boosts by making it switch out (and, like Chancey, it isn't 2HKOed by Geomancy Sylveon after SR)
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 0 HP / 0 Def Sylveon: 458-542 (138.3 - 163.7%) -- guaranteed OHKO
224+ Atk Mega Sharpedo Poison Jab vs. 0 HP / 0 Def Sylveon: 282-334 (85.1 - 100.9%) -- 6.3% chance to OHKO (87.5% chance to OHKO after SR)
(M-Sharpedo, after one Speed Boost, outspeeds and can OHKO Geomancy Sylveon, but if it's Timid instead of Modest, then...)
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0- SpD Mega Sharpedo: 1070-1262 (380.7 - 449.1%) -- guaranteed OHKO (ouch!)
Empoleon can use Yawn to either make Sylveon asleep or force it to lose all of its boosts by making it switch out (and, like Chancey, it isn't 2HKOed by Geomancy Sylveon after SR)
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 0 HP / 0 Def Sylveon: 458-542 (138.3 - 163.7%) -- guaranteed OHKO
224+ Atk Mega Sharpedo Poison Jab vs. 0 HP / 0 Def Sylveon: 282-334 (85.1 - 100.9%) -- 6.3% chance to OHKO (87.5% chance to OHKO after SR)
(M-Sharpedo, after one Speed Boost, outspeeds and can OHKO Geomancy Sylveon, but if it's Timid instead of Modest, then...)
+2 252 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 0- SpD Mega Sharpedo: 1070-1262 (380.7 - 449.1%) -- guaranteed OHKO (ouch!)
Granted that SR aren't up, Amoonguss survives a boosted Psyshock and can retaliate with Clear Smog to get rid of Geomancy Sylveon's boosts
4 Atk Bronzong Gyro Ball (150 BP) vs. 0 HP / 0 Def Sylveon: 420-494 (126.8 - 149.2%) -- guaranteed OHKO (Bronzong is only 2HKOed at best by HP Fire and, if it has HP Ground instead, 3HKOed at best by Hyper Voice while it cleanly OHKOes Geomancy Sylveon)
4 Atk Bronzong Gyro Ball (150 BP) vs. 0 HP / 0 Def Sylveon: 420-494 (126.8 - 149.2%) -- guaranteed OHKO (Bronzong is only 2HKOed at best by HP Fire and, if it has HP Ground instead, 3HKOed at best by Hyper Voice while it cleanly OHKOes Geomancy Sylveon)
If Sylveon has HP Fire, it's walled by Heatran and Chansey
If Sylveon has HP Ground, it's walled by M-Scizor, Ferrothorn, Chansey and Bronzong (using Shadow Ball can alleviate its matchup against certain mons like Heatran or Bronzong) and I've probably forgot some mons for one/both of the HPs
I didn't include Sash leads like Mamoswine, Scolipede and Azelf since they're used as dedicated leads and by the time Sylveon sets up, they'll either be already KOed or having their Sash broken (or won't be at full HP at least)
As we can see, there's only a couple of mons that can stop this monster and the right conditions must be met to do so or most of them will be KOed. Moreover, most of the listed mons are Megas or very passive, which means the number of Geomancy Sylveon checks will be very limited within a team and thusly makes it a very centralising (to not say broken) threat in the metagame.
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