Posting sets that defensive Pokemon use for the format of Gen 9 NU:
Bronzong @ Leftovers
Ability: Levitate
Tera Type: Fairy
EVs: 252 HP / 164 Def / 92 SpD
Sassy Nature
- Stealth Rock
- Psychic Noise
- Body Press / Earthquake
- Heavy Slam / Iron Defense / Gyro Ball
Bronzong's defensive stats, typing, and access to Levitate makes it a good defensive Pokemon for Gen 9 NU, being able to check Flygon. Psychic Noise can shut down HP recovery of opposing Pokemon, and Bronzong can even be a wincon if you use Iron Defense + Body Press. Gyro Ball is an alternate Steel STAB option to be able handle Dragon Dance / Choice Scarf Flygon better.
Registeel @ Leftovers
Ability: Clear Body
Tera Type: Ghost / Water
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Stealth Rock
- Heavy Slam
- Earthquake
- Thunder Wave / Ice Spinner
Registeel's bulk makes it a good defensive Pokemon, being able to check special wallbreakers such as Porygon-Z, Kilowattrel, Munkidori, Galarian Articuno, and Sylveon. Earthquake lets Registeel hit Chandelure, Incineroar, Toxtricity and Hisuian Typhlosion super effectively. Thunder Wave lets Registeel cripple faster Pokemon, being a way for Registeel to check faster Pokemon. Ice Spinner is an option to discourage Flygon from setting up on it. Tera Ghost gives Registeel a Fighting immunity to check the Fighting-types in the tier, though you can use Water for a nice defensive typing to check Chandelure and Hisuian Typhlosion.
Slowbro-Galar @ Rocky Helmet
Ability: Regenerator
Tera Type: Water
EVs: 252 HP / 228 Def / 28 Spe
Bold Nature
- Sludge Bomb
- Flamethrower
- Calm Mind
- Slack Off
Galarian Slowbro's typing and ability Regenerator makes it a solid Fighting-type check against Mienshao, Waterbull (Tauros-Paldea-Aqua), Toxicroak, Firebull (Tauros-Paldea-Blaze), and Breloom. Galarian Slowbro is also nice to check other physical wallbreakers such as Scyther, Cinccino, and Basculegion if Galarian Slowbro used its Tera. Galarian Slowbro can also serve as a wincon late-game with Calm Mind to boost its offensive power while letting it tank special attacks better. Tera Water gives Galarian Slowbro a nice defensive typing to check wallbreakers such as Basculegion, Flygon, and Waterbull better. Rocky Helmet lets Galarian Slowbro force chip damage against Pokemon who use a contact move on it.
Vaporeon @ Heavy-Duty Boots / Leftovers
Ability: Water Absorb
Tera Type: Poison
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Scald
- Wish
- Protect
- Flip Turn / Haze / Ice Beam / Alluring Voice
Vaporeon is a good defensive Pokemon thanks to its bulk and Water Absorb ability that allow it to check offensive Water-types such as Basculegion, Inteleon, Waterbull, and Araquanid. Vaporeon can also check other physical wallbreakers with Scald's chance to burn opposing Pokemon. Vaporeon also provides utility for its team with Wish to heal its teammates and itself. Protect is nice to scout for choice locked Pokemon and can let Vaporeon get the HP recovery from Wish if it used Wish the previous turn. Flip Turn lets Vaporeon provide more utility by being a pivot to potentially Wishpass to its teammates. Haze can be used to get rid of stat boosts from opposing Pokemon, Ice Beam targets Grass-types such as Brambleghast, Vileplume, Tsareena, and Breloom, and Alluring Voice can be used to deal super effective damage to Waterbull. Tera Poison is used for a defensive typing to be able to check Waterbull better.
Milotic @ Heavy-Duty Boots
Ability: Marvel Scale / Competitive
Tera Type: Fairy
EVs: 252 HP / 252 Def / 4 Spe
Bold Nature
- Scald
- Recover
- Flip Turn / Dragon Tail / Alluring Voice
- Ice Beam / Haze / Alluring Voice
Milotic is another good defensive Water-type for Gen 9 NU, being able to utilize Marvel Scale to get a defense boost if statused by burns, paralysis, and poison, or Competitive to get a boost to its Special Attack due to how common Intimidate is in the tier. Scald and Recover are mandatory on Milotic, being used to cripple opposing Pokemon with burns and have reliable HP recovery, respectively. The last two slots can be mixed and match of the following moves: Flip Turn, Dragon Tail, Ice Beam, Haze, and Alluring Voice. Flip Turn lets Milotic generate/keep up momentum for its team. Dragon Tail/Haze is used to deal with setup sweepers by forcing them out the field/removing their stat boosts. Ice Beam targets the Grass-types in the tier, while Alluring Voice hits Aqua Tauros super effectively. Tera Fairy allows Milotic to have a type that lets it check Fighting-types such as Mienshao and Aqua Tauros.
Gastrodon @ Leftovers / Heavy-Duty Boots
Ability: Storm Drain
Tera Type: Ghost
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
IVs: 0 Atk
- Earth Power
- Ice Beam / Sludge Bomb / Surf
- Recover
- Stealth Rock / Spikes
The last defensive Pokemon I'll be covering is Gastrodon. Gastrodon's typing and bulk is nice to check special wallbreakers, with Storm Drain allowing Gastrodon to switch into Water moves. Ice Beam is used to hit Grass-types such as Brambleghast, Tsareena, and Breloom. Sludge Bomb is an option to cripple opposing bulky Water-types with the poison chance while still being able to hit Tsareena and Breloom super effectively. Surf can be used if you prefer to have a Water STAB. Stealth Rock or Spikes lets Gastrodon provide hazards for its teammates, depending on which hazards your team needs. Tera Ghost gives Gastrdon a Fighting immunity, being useful against Choice locked Pokemon that are locked into using a Fighting (or Normal) move. Tera Ghost is also useful to potentially blank Aqua Tauros depending on its set.