The upcoming Monotype OM tour is a nice way to attract attention to these tiers.
But, before the tour starts I feel like we should really address the following problem in Monotype Ubers first:
Xerneas @ Fairium Z
Ability: Fairy Aura
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Geomancy
- Ingrain
- Rest
- Moonblast
Z-Geo Xern is so borked it influences the whole tier by just being there (in addition it's not just there, but also on a type that is already amazing without it).
Z-Geomancy allows Xerneas to comfortably run a defensive Rest set, since after completing it's ritual it will be at +1 Defense and +3 Special Defense. It also can't be phazed out due to Ingrain. Especially the +1 Defense is a gamebreaker.
Hardly anything except for Physical STAB Poison and Steel moves can touch this thing, and the attackers almost always have to switch in immediately and can't beat an already boosted Xern.
There's only a handful of (relevant) Pokemon that don't get 2HKO'd by a +3 Moonblast which include SpDef Pdon, Ho-Oh, Toxapex, VenusaurM, (High) SpDef ScizorM, Ferrothorn, Heatran, Celesteela and Aegislash. Five of which are Steel types (of whom 3 still don't have a quick way of taking Xern out alone) and all the others can't touch it without crits.
And to think all I said in this post was without taking into consideration Klefki's Screens which makes everything so much worse.
Now, I'm not the best writer or arguer on the forums but I hope I've brought up enough to back my opinion that Fairium Z alongside Geomancy should be banned on Xerneas as soon as possible. If I missed any important details or if I am using questionable arguments, please do correct me.
But, before the tour starts I feel like we should really address the following problem in Monotype Ubers first:

Xerneas @ Fairium Z
Ability: Fairy Aura
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Geomancy
- Ingrain
- Rest
- Moonblast
Z-Geo Xern is so borked it influences the whole tier by just being there (in addition it's not just there, but also on a type that is already amazing without it).
Z-Geomancy allows Xerneas to comfortably run a defensive Rest set, since after completing it's ritual it will be at +1 Defense and +3 Special Defense. It also can't be phazed out due to Ingrain. Especially the +1 Defense is a gamebreaker.
Hardly anything except for Physical STAB Poison and Steel moves can touch this thing, and the attackers almost always have to switch in immediately and can't beat an already boosted Xern.
240+ Atk Groudon-Primal Precipice Blades vs. +1 248 HP / 252+ Def Xerneas: 136-162 (29.8 - 35.6%) -- guaranteed 4HKO after Leftovers recovery
0 Atk Steelix-Mega Heavy Slam (80 BP) vs. +1 248 HP / 252+ Def Xerneas: 104-126 (22.8 - 27.6%) -- possible 5HKO after Leftovers recovery
252 Atk Hoopa-Unbound Gunk Shot vs. +1 248 HP / 252+ Def Xerneas: 152-180 (33.4 - 39.5%) -- 18.1% chance to 3HKO after Leftovers recovery
0 SpA Venusaur-Mega Sludge Bomb vs. +3 248 HP / 8 SpD Xerneas: 96-114 (21 - 25%) -- possible 5HKO after Leftovers recovery
252+ SpA Venusaur-Mega Sludge Bomb vs. +3 248 HP / 8 SpD Xerneas: 126-150 (27.6 - 32.9%) -- 77.3% chance to 4HKO after Leftovers recovery
240+ Atk Muk-Alola Gunk Shot vs. +1 248 HP / 252+ Def Xerneas: 186-218 (40.8 - 47.9%) -- guaranteed 3HKO after Leftovers recovery
0 Atk Toxapex Poison Jab vs. 248 HP / 252+ Def Xerneas: 90-108 (19.7 - 23.7%) -- possible 5HKO
0 SpA Toxapex Sludge Bomb vs. 248 HP / 8 SpD Xerneas: 116-140 (25.4 - 30.7%) -- 3.7% chance to 4HKO after Leftovers recovery
(Haze all you want but you're not doing anything in return)
208+ Atk Life Orb Ho-Oh Brave Bird vs. +1 248 HP / 252+ Def Xerneas: 136-161 (29.8 - 35.3%) -- guaranteed 4HKO after Leftovers recovery
(Last but not least, everyone's favorite bird)
The only non-Steel type that can switch in on an unboosted Xern and always take it out is probably this, and only if it gets the right rolls or if it's health is higher than 85%.
252 SpA Gengar-Mega Sludge Wave vs. +1 248 HP / 8 SpD Xerneas: 254-302 (55.8 - 66.3%) -- guaranteed 2HKO
252 SpA Gengar-Mega Sludge Wave vs. +3 248 HP / 8 SpD Xerneas: 152-182 (33.4 - 40%) -- guaranteed 3HKO
+3 0 SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 0 SpD Gengar-Mega: 222-262 (85 - 100.3%) -- 6.3% chance to OHKO
And even the strong physical Steel types struggle with killing this thing. Metagross and Scizor also only win if they switch in immediately when Xerneas comes out unboosted.
252 Atk Tough Claws Metagross-Mega Meteor Mash vs. +1 248 HP / 252+ Def Xerneas: 206-246 (45.2 - 54%) -- 2.3% chance to 2HKO after Leftovers recovery
+2 108+ Atk Technician Scizor-Mega Bullet Punch vs. +1 248 HP / 252+ Def Xerneas: 218-258 (47.9 - 56.7%) -- 28.9% chance to 2HKO after Leftovers recovery
4 Atk Ferrothorn Gyro Ball (150 BP) vs. +1 248 HP / 252+ Def Xerneas: 152-182 (33.4 - 40%) -- 27.9% chance to 3HKO after Leftovers recovery
0 Atk Aegislash-Blade Gyro Ball (104 BP) vs. +1 248 HP / 252+ Def Xerneas: 158-188 (34.7 - 41.3%) -- 68.7% chance to 3HKO after Leftovers recovery
0 Atk Celesteela Heavy Slam (100 BP) vs. +1 248 HP / 252+ Def Xerneas: 110-132 (24.1 - 29%) -- possible 5HKO after Leftovers recovery
0 Atk Steelix-Mega Heavy Slam (80 BP) vs. +1 248 HP / 252+ Def Xerneas: 104-126 (22.8 - 27.6%) -- possible 5HKO after Leftovers recovery
252 Atk Hoopa-Unbound Gunk Shot vs. +1 248 HP / 252+ Def Xerneas: 152-180 (33.4 - 39.5%) -- 18.1% chance to 3HKO after Leftovers recovery
0 SpA Venusaur-Mega Sludge Bomb vs. +3 248 HP / 8 SpD Xerneas: 96-114 (21 - 25%) -- possible 5HKO after Leftovers recovery
252+ SpA Venusaur-Mega Sludge Bomb vs. +3 248 HP / 8 SpD Xerneas: 126-150 (27.6 - 32.9%) -- 77.3% chance to 4HKO after Leftovers recovery
240+ Atk Muk-Alola Gunk Shot vs. +1 248 HP / 252+ Def Xerneas: 186-218 (40.8 - 47.9%) -- guaranteed 3HKO after Leftovers recovery
0 Atk Toxapex Poison Jab vs. 248 HP / 252+ Def Xerneas: 90-108 (19.7 - 23.7%) -- possible 5HKO
0 SpA Toxapex Sludge Bomb vs. 248 HP / 8 SpD Xerneas: 116-140 (25.4 - 30.7%) -- 3.7% chance to 4HKO after Leftovers recovery
(Haze all you want but you're not doing anything in return)
208+ Atk Life Orb Ho-Oh Brave Bird vs. +1 248 HP / 252+ Def Xerneas: 136-161 (29.8 - 35.3%) -- guaranteed 4HKO after Leftovers recovery
(Last but not least, everyone's favorite bird)
The only non-Steel type that can switch in on an unboosted Xern and always take it out is probably this, and only if it gets the right rolls or if it's health is higher than 85%.
252 SpA Gengar-Mega Sludge Wave vs. +1 248 HP / 8 SpD Xerneas: 254-302 (55.8 - 66.3%) -- guaranteed 2HKO
252 SpA Gengar-Mega Sludge Wave vs. +3 248 HP / 8 SpD Xerneas: 152-182 (33.4 - 40%) -- guaranteed 3HKO
+3 0 SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 0 SpD Gengar-Mega: 222-262 (85 - 100.3%) -- 6.3% chance to OHKO
And even the strong physical Steel types struggle with killing this thing. Metagross and Scizor also only win if they switch in immediately when Xerneas comes out unboosted.
252 Atk Tough Claws Metagross-Mega Meteor Mash vs. +1 248 HP / 252+ Def Xerneas: 206-246 (45.2 - 54%) -- 2.3% chance to 2HKO after Leftovers recovery
+2 108+ Atk Technician Scizor-Mega Bullet Punch vs. +1 248 HP / 252+ Def Xerneas: 218-258 (47.9 - 56.7%) -- 28.9% chance to 2HKO after Leftovers recovery
4 Atk Ferrothorn Gyro Ball (150 BP) vs. +1 248 HP / 252+ Def Xerneas: 152-182 (33.4 - 40%) -- 27.9% chance to 3HKO after Leftovers recovery
0 Atk Aegislash-Blade Gyro Ball (104 BP) vs. +1 248 HP / 252+ Def Xerneas: 158-188 (34.7 - 41.3%) -- 68.7% chance to 3HKO after Leftovers recovery
0 Atk Celesteela Heavy Slam (100 BP) vs. +1 248 HP / 252+ Def Xerneas: 110-132 (24.1 - 29%) -- possible 5HKO after Leftovers recovery
Only Poison, Ghost, Fire, Grass, Bug, Psychic, Flying and Steel have checks to this Xerneas set.
Poison and Grass are D ranked in the viability rankings and would mainly be used to check Xerneas. In addition Poison's best check, Gengar-Mega, should preferably have been mega evolved already earlier in the match to check Xern. A healthy Muk-Alola can also check Xerneas (if it comes in immediately) if it lands it's Gunk shots. But Poison still has to watch out for Magearna and MawileM. Grass does counterteam Fairy really nicely with Kartana and Ferrothorn, though.
Ghost only has Aegislash, which will be pressurised a lot more during the Fairy matchup than only by Xerneas. Or it will have to give up Mega Sableye for Mega Gengar alongside Aegislash to strengthen the Fairy matchup.
(Not necessarily a bad swap since Ghost is perfectly able to be offensive in Ubers)
Fire has Heatran who takes pretty much 0 damage from a Moonblast, but it can't take Xerneas out without critting Flash cannons.
EDIT: I forgot to mention Quiver Dance Roost Bulkarona, that thing Counters.
Bug has the most reliable check in Scizor, BUT if a Tapu lele set a Psychic Terrain for Xerneas, Scizor literally won't even be able to touch Xerneas which turns the tables around completely.
Psychic has Metagross-Mega, but Meteor Mash doesn't guarantee a 2HKO on +1 Defense. So Metagross will have to stay healthy throughout the match (above 60%) and HAS to switch in every time Xern comes in unboosted.
Flying has Celesteela which can click Leech Seed + Heavy slam to stall out the PP of both Pokemon, but isn't a check if you're looking for a battle with less than 100 turns.
Steel is Steel. No problem here.
Tl;dr only praying for critical hits, Grass, Poison (With Mega Gengar), Ghost (with at least Aegislash and optionally Mega Gengar), Fire and Steel have a win-able matchup against Fairy monotype teams. The first two being D ranked types.
Poison and Grass are D ranked in the viability rankings and would mainly be used to check Xerneas. In addition Poison's best check, Gengar-Mega, should preferably have been mega evolved already earlier in the match to check Xern. A healthy Muk-Alola can also check Xerneas (if it comes in immediately) if it lands it's Gunk shots. But Poison still has to watch out for Magearna and MawileM. Grass does counterteam Fairy really nicely with Kartana and Ferrothorn, though.
Ghost only has Aegislash, which will be pressurised a lot more during the Fairy matchup than only by Xerneas. Or it will have to give up Mega Sableye for Mega Gengar alongside Aegislash to strengthen the Fairy matchup.
(Not necessarily a bad swap since Ghost is perfectly able to be offensive in Ubers)
Fire has Heatran who takes pretty much 0 damage from a Moonblast, but it can't take Xerneas out without critting Flash cannons.
EDIT: I forgot to mention Quiver Dance Roost Bulkarona, that thing Counters.
Bug has the most reliable check in Scizor, BUT if a Tapu lele set a Psychic Terrain for Xerneas, Scizor literally won't even be able to touch Xerneas which turns the tables around completely.
Psychic has Metagross-Mega, but Meteor Mash doesn't guarantee a 2HKO on +1 Defense. So Metagross will have to stay healthy throughout the match (above 60%) and HAS to switch in every time Xern comes in unboosted.
Flying has Celesteela which can click Leech Seed + Heavy slam to stall out the PP of both Pokemon, but isn't a check if you're looking for a battle with less than 100 turns.
Steel is Steel. No problem here.
Tl;dr only praying for critical hits, Grass, Poison (With Mega Gengar), Ghost (with at least Aegislash and optionally Mega Gengar), Fire and Steel have a win-able matchup against Fairy monotype teams. The first two being D ranked types.
Now, I'm not the best writer or arguer on the forums but I hope I've brought up enough to back my opinion that Fairium Z alongside Geomancy should be banned on Xerneas as soon as possible. If I missed any important details or if I am using questionable arguments, please do correct me.
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