Corsoul

Corsoul is a great defensive Pokemon thanks to its typing and ability in Liquid Scales, which allows it to stay healthy and reduce the chip damage it takes from weaker moves. This longevity and typing coupled with its defensive stats, it is able to constantly keep Stealth Rock up and check many Wallbreakers. Corsoul's weaknesses in Dark and Ghost are common attacking types however, so it can be threatened by any Pokemon. Furthermore, its passivity can exploited pretty easily.

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Defensive Stealth Rock
Corsoul @ Leftovers / Colbur Berry
Ability: Liquid Scales
EVs: 252 HP / 252 Def / 4 SpD
Relaxed Nature
- Poltergeist / Hex
- Scald / Will-O-Wisp
- Strength Sap
- Stealth Rock

This is the go-to Corsoul set, with its defensive investment and the combination of either Scald or Will-O-Wisp and Strength Sap it is able to check a majority of the Physical Attackers. Scald is better against Substitute Pokemon and it doesn't allow Fire-types and Ground-types to abuse its presence but Will-O-Wisp is more likely to hit than Scald can burn. Poltergeist is a really strong move for a defensive Pokemon, but itemless Pokemon can switch into that easily, so Hex is an option over that. Its good natural special bulk makes it decent against some Special Attackers as well.

Morpeko-Marsh is a great partner for Corsoul since they cover each others weaknesses pretty well and Morpeko-Marsh also provides Hazard Control with Rapid Spin and Spikes. Ground-types in Groudon, Sandslash-Lustrous, Zygarde-Goliath and Landorus-Bengal free it up from a Stealth Rock role and provide an Electric immunity while Corsoul can act as a backbone for them while they break past other Walls. Rotom-Fan removes Hazards and is able to check all Ground-types while being able to check Deoxys-Gem, Silvally-Fairy and Landorus which would otherwise overwhelm Corsoul

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Bulk Up Sweeper
Corsoul @ Leftovers / Colbur Berry
Ability: Liquid Scales
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Poltergeist
- Liquidation / Earthquake / Will-O-Wisp
- Bulk Up
- Strength Sap

Corsoul's great special bulk and Liquid Scales gives it many opportunities to set-up with Bulk Up. Bulk Up and Strength Sap give it a great options to 1v1 Physical Attackers and thus make it good defensive backbone. Poltergeist is a good STAB move and Liquidation or Earthquake gives it good coverage, however itemless Pokemon that don't mind taking a Water or Ground move take advantage of it easily. Will-O-Wisp is also a good move to use but being unable to to damage Normal- and Dark-types is troublesome. Draining and Stalling Pokemon is good however and thus it is an option.

Toxic Spikes helps Corsoul with its Dark-types problem and it cripples other defensive Calm Mind Pokemon like Corsola, Deoxys-Tank, Exeggutor, Kyogre and Meloetta too. Hazard Support from Ground-types or Silvally formes is much appreciated since they also build solid defensive cores or break holes through teams, for Corsoul to abuse.

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Calm Mind Sweeper
Corsoul @ Leftovers / Colbur Berry
Ability: Liquid Scales
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
IVs: 0 Atk
- Shadow Ball / Hex
- Scald / Will-O-Wisp
- Calm Mind
- Strength Sap

Similar to its Bulk Up counterpart, Calm Mind sets can set-up in many situations but unlike Bulk Up, its STABs are more consistant. Shadow Ball has a chance to snowball more easily, when Corsoul has already reached +6 in Special Attack. Hex and Scald or Will-O-Wisp however have a great synergy on this set. Will-O-Wisp has a more immediate impact, but Scald does provide good coverage for Normal- and Dark-types and unlike Liquidation, it can cripple those switch-ins.

It does have similar team structures to the Bulk Up set, so Toxic Spikes can help it break past defensive Pokemon or Calm Mind users, while Ground-types and Silvally make good partners due to their defensive synergy, Hazard Support and wallbreaking potential.

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Other Options

Defensive Stealth Rock: Haze is a decent option, which doesn't allow Pokemon to set-up in Corsoul's face. However, Corsoul's moveset is already filled to the brim, so fitting it becomes an issue.

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Checks und Counter

Raticate: Raticate is immune to Ghost and both of its abilities, those being Guts and Burn Heal, make it not fear Burn from Will-O-Wisp or Scald. It can worn down, but Corsoul has to fear potential Pursuits. Bulk Up sets have potential to 1v1 Raticate provided that Raticate has either no super effective move or Pursuit to hit Corsoul. Throat Chop or Thunder Punch are troublesome for it.

Dark-types: Dark-types are extremely common in the metagame, thus almost every team packs a built in check for Corsoul. Darmanitan, Dugtrio, Raticate-Alola and Morpeko-Marsh are strong Wallbreakers and the former doesn't fear a potential burn. Linoone-Galar has strong Special Attacks while Silvally-Dark and Muk-Oilslick are ruinous Pursuit trappers. Weezing-King is a downhill match-up since it removes Liquid Scales.

Taunt: Taunt shuts down a majority of Corsoul's existance and the biggest problem are the Dark-types who are the most common Taunt users. Furthermore, Pokemon that it is suppose to check like Lycanroc-Dusk, Darmanitan-Galar, Keldeo and Deoxys-Gem can Taunt Corsoul and beat it easily.