Flutter Mane:
Choice Specs Flutter Mane generally runs Pixilate Boomburst sets with Tera Fairy. Both Timid and Modest see use. Alternatively, Flare Boost sets with Nasty Plot see use; these carry Tera Ghost or Tera Steel, and are always Timid.
Sets here.
Choice Specs Flutter Mane misses most 2HKOes on standard walls like Ice Scales Zacian, Ice Scales Dialga-O, and Ice Scales Arceus formes. Tera Fairy fixes this issue with the damage buff. Zacian takes 52.3%-61.5% from Timid Tera (2HKOed with one hazard, even after Spirit Break), and Modest Tera deals 57.2%-67.5% (basically guaranteed 2HKO). Dialga-O takes 44.3%-52.2% from Timid Tera (guaranteed OHKO after one Spike and favoured with Stealth Rock), and Modest Tera is basically guaranteed to 2HKO. Arceus fares the best but still takes 40.3%-47.5% from Timid Tera (53% to 2HKO with one hazard) and 44.3%-52.2% from Modest Tera (unlikely 2HKO without hazards, guaranteed with). The TL;DR here is that
Flutter Mane "counters" only beat Tera Fairy Flutter Mane with their own Tera.
Flare Boost is a less clear-cut Tera-abusing Breaker thanks to needing to set up to deal notable damage. Zacian checks it with Haze and Spirit Break but still takes 37.3%-44% from Tera Ghost Astral Barrage on the switch. Ice Scales Arceus has a harder time thanks to needing to 1v1 Nasty Plot, and if Arceus switches in on Nasty Plot, it simply gets KOed unless it Calm Minds on the first hit and Flutter Mane gets 2 low rolls. Dialga-O fares worse, being 2HKOed by +2 Tera Ghost Astral Barrage and only being able to fight back if it has Gigaton Hammer.
As for Tera Steel, it offers no relevant damage amplification, but it allows Flutter Mane to 1v1 all of these in the long run thanks to resisting basically all of their attacks (spare the rare Earth Plate Judgment on Arceus formes like Arceus-Poison).
Rayquaza:
Rayquaza runs a mixed Aerilate set with Boomburst, Extreme Speed, a coverage move (Collision Course, Headlong Rush, or V-create), and a recovery move (Strength Sap or Jungle Healing). It is always Tera Flying and almost always Rash (Naive is occasionally used specifically to Speed-creep other Rayquaza).
Set here.
Rayquaza hits hard even before Tera. Most specially defensive checks, like RegenVest and Chansey, don't work thanks to being mixed; Chansey is potentially OHKOed by Collision Course and 2HKOed by Extreme Speed, Dialga-O is 2HKOed by all physical coverage moves, and Ting-Lu is 2HKOed by both Boomburst and Collision Course. This leaves basically just Ice Scales Arceus as a common special wall that handles Rayquaza, as Zacian
can but gets dangerously close to being 2HKOed by Extreme Speed. That's where Tera Flying comes in, as Ice Scales Arceus and Zacian are 2HKOed by Tera Flying Extreme Speed, and common Flying resists like Corviknight and Dialga-O are simply 2HKOed by Boomburst (the latter without Assault Vest or Ice Scales). This leaves basically 3 "consistent" counterplay options in Ice Scales Arceus-Electric, specially defensive Corviknight, and Soundproof users like Dondozo and Giratina; the former two can simply fold to Rayquaza's coverage and are heavily flawed Pokemon otherwise, and those Soundproofers are both flawed (they don't beat the other Boomburst users) and can be 2HKOed by Extreme Speed with one hazard up.
Koraidon:
Koraidon runs a Choice Band set with Sword of Ruin, dual STABs, Gigaton Hammer, and Flip Turn. It is always Tera Dragon and always Jolly. Rarely, it runs Mold Breaker.
Set here.
Koraidon generally doesn't hit very good ranges without Tera, as no relevant Fur Coat users are 2HKOed by Glaive Rush; Koraidon often relies on racking up hazard and Flip Turn chip before going for KOes (which it does with relative ease). For Mold Breaker sets, it 2HKOes Fur Coat users without any prior chip, but has lower damage output into naturally bulky non-Fur Coat users like Dondozo. Tera Dragon enables Glaive Rush to 2HKO Fur Coat Arceus formes and Groudon after one Flip Turn or entry hazard on Sword of Ruin sets, prevents Rayquaza from revenge killing it, and allows Mold Breaker sets to more reliably break past non-Prankster Dondozo.
Regieleki:
Regieleki runs a Choice Specs + Hadron Engine set with Rising Voltage, Volt Switch, Ice Beam, and U-turn. It is always Modest and Tera Electric. It also usually runs minimum Special Defense to maximize damage on Imposter.
Set here.
Regieleki hits a lot of good ranges without Tera. It always 2HKOes Ice Scales Zacian and can, albeit rarely, 2HKO Ice Scales Arceus with Stealth Rock. Minimum SpDef variants always 2HKO Imposter with Rising Voltage without Tera. With Tera, its only good checks become Ground-types, as Zacian takes over 2/3 from Rising Voltage, Arceus is always 2HKOed, Imposter can rarely be OHKOed, and even Chansey can be 2HKOed.
Arceus-Ghost:
Arceus-Ghost has two main sets, both of which use Spooky Plate + Judgment and Fighting coverage. Fur Coat sets use Nasty Plot and Focus Blast, while Simple sets use No Retreat and Collision Course.
Sets here.
The main differences between the sets, at least for the purposes of this analysis, are that Nasty Plot can be spammed, and Simple means Spirit Break and Parting Shot lower Special Attack by 2 stages. +2 Judgment excels at picking off most offensive options in the tier, with Fighting coverage chunking Dialga-O and Ting-Lu (in Collision Course's case, Chansey too) for hefty damage. These often fall short at Ice Scales Zacian and opposing Ice Scales Arceus, however; Ice Scales Zacian completely shuts both down with Haze and Spirit Break, and Arceus-Ghost can 1v1 both with Calm Mind boosts. For Simple sets, Imposter Chansey can also reasonably stall a good amount of Judgment PP thanks to No Retreat's Special Defense boosts. With Tera Ghost, Arceus-Ghost can prevent Imposter from PP stalling it, Judgment hits every relevant Prankster user for over half (preventing them from safely switching in and making them much more susceptible to running out of Recover PP), Zacian can no longer safely PP stall Judgment, and Nasty Plot sets have the potential to 1v1 Ice Scales Arceus (barely falling short of a 2HKO).
Palkia-O:
Palkia-O generally runs Nasty Plot sets with either Speed Boost, Mold Breaker, or Simple. These have Dragon Energy, Steam Eruption, and Strength Sap. They can run all of Tera Dragon, Tera Water, and Tera Fairy. Speed Boost sets are Modest, while the others are Timid.
Sets here.
Palkia-O is very dangerous by default, although even with Simple it falls short of most OHKOes it wants, and Dragon typing + Dragon Energy's HP reliance don't do it many favours defensively. Simple Dragon Energy is capable of OHKOing RegenVest Dialga-O at +4, with Mold Breaker picking off Ice Scales Dialga-O at +2. Both sets deal hefty damage to Ice Scales Arceus-Fairy (+2 Moldy Steam and +4 Simple Steam both 2HKO, but Simple does 51.3%-60.3% while Moldy does 68.4%-80.6%) and Zacian (Simple does 66.4%-78%, Moldy does 88.4%-104.3%), but it must also contend with their Fairy-type moves. That's where Tera Water comes in, as it both buffs damage and removes the ability to scare Palkia-O offensively, enabling Simple to hit both of Moldy's ranges and allowing Moldy to OHKO both (Zacian guaranteed). Alternatively, Tera Dragon allows Dragon Energy to OHKO Dialga-O on both sets and also allows Simple to OHKO Ice Scales Arceus (non-Fairy) with Dragon Energy. Speed Boost is the least threatening of all of these on raw damage, but Tera Water still gives it the kick to 2HKO Zacian, meaning it wins in the long run.
Eternatus:
Eternatus usually runs Mold Breaker. It generally aims to take advantage of Eternatus' good bulk and Poison STAB, matching well into the most popular special walls in the Fairies. It can be Tera Fire, Tera Dragon, or Tera Fairy (in descending popularity).
Set here.
Mold Breaker Eternatus is mostly used for how well it can harass the Fairy-type walls, always winning in the long term thanks to Torch Song boosts and possibly Poison STAB. Dragon Energy is still potent, however, especially when boosted; +0 Dragon Energy can 2HKO Ice Scales Dialga-O, +1 2HKOes RegenVest Dialga-O and Ting-Lu, and Torch Song provides a general tool to prevent Prankster users from staying in and stalling Dragon Energy. Tera Dragon amplifies this by allowing +2 Dragon Energy to OHKO RegenVest Dialga-O and Ting-Lu, and for +1 Dragon Energy to OHKO Ice Scales Dialga-O, whereas Tera Fire basically guarantees Eternatus will always 1v1 the Fairy-types without any inconvenience. Tera Fairy provides an almost completely safe Imposter-proofing method.
Calyrex-I:
Calyrex-I uses a Sword of Ruin set with Swords Dance, Fire/Ice coverage, and Tera Fire. Generally paired with support for Water-types like Dondozo.
Set here.
Caly generally gets some good ranges into Fur Coat users like Arceus and Tera Groudon (which usually Terastallizes when seeing Caly-I or Palafin-H at preview), with +2 Glacial Lance doing up to 80% on both and having the bulk to reliably brawl with them. +2 V-create also nearly OHKOes Imposter. It's scared out by Imposter, however, as the Speed-tie means the winner eliminates the other. This is where Tera Fire comes in; it allows Calyrex-I to make the Imposter matchup completely one-sided and also allows it to OHKO those Fur Coat users with V-create.
Iron Bundle:
Iron Bundle runs a Choice Specs Refrigerate set with Tera Ice. It's generally the weakest of the main wallbreakers in the meta, but makes up for this with its Speed, letting it outspeed Koraidon and Flutter Mane. Due to the Speed advantage being the main reason to use it, it's always Timid.
Set here.
Iron Bundle generally hits similar ranges to Timid Choice Specs Flutter Mane, albeit lower. It's much more reliant on hazards for 2HKOes, which are required to 2HKO Dialga-O with Tera Ice and 2HKO Zacian with Tera Ice after Spirit Break. Ice Scales Arceus requires multiple layers of hazards or a lot of prior chip to be 2HKOed.