NPC intimidator required.

Decidueye-Hisui @ Damp Rock
Ability: Overgrow
Tera Type: Grass
EVs: 68 HP / 252 Def / 188 SpA
Bold Nature
Giga Drain
Nasty Plot
Rain Dance
Taunt

Vitamins: 6 HP Up (or Health Mochi), 25 Iron (or Resist Mochi), 18 Calcium (or Genius Mochi)
Feathers: 8 Health Feathers, 2 Resist Feathers, 8 Genius Feathers

Stats at lvl 100 w/ perfect IVs:
HP : 334
Defense : 284
Sp.Atk. : 273
Sp.Def.: 226
Speed : 156

NPC Advantages: 1 intimidator (tauros, staraptor, arcanine) required, because decidueye takes too much damage from flare blitz and has no way to cancel swords dance (without clearing its own buffs). gardevoir (life dew, synchronize for burns), bellibolt (discharge, light screen), umbreon (thunder wave, synchronize for burns), drifblim (will-o-wisp), sylveon (charm, moonblast), arboliva (seed sower), and dudunsparce (thunder wave) are helpful.

Strategy: rain dance (renewing when it expires), wait for orb steal, 2-3 giga drain, taunt, giga drain, wait until infernape resets its stats. terastalize. giga drain when you need HP. 2-3 nasty plot. giga drain to victory.

Notes: physical or fighting tera decidueye would work in duos with support, but solo it's not feasible due to lack of healing (if it could learn horn leech or drain punch it would work). shell bell and roost/synthesis are not an option because the damp rock, rain dance, and taunt are essential to survival. perhaps with heavy reliance on NPC gardevoir and good RNG heal cheers it could be done, but I don't recommend it.

Pros: supereffective tera type matching base type, decent HP and special defense stats. great defense and special attack stats. resists grass knot, resists tera blast before terastalizing, nasty plot buffs 2 special attack stages, rain dance puts up rain for 8 turns (extended from 5 by damp rock, which is critical because decidueye gets outsped), which halves damage from supereffective flare blitz. taunt blocks infernape from using nasty plot. giga drain has 75 base power, 100 accuracy, and deals supereffective STAB (tera-boosted when terastalized) damage. it heals an amount equal to 50% of the damage it deals. overgrow gives 1.5x boost to giga drain when HP is below 1/3.

Cons: weak to flare blitz, outsped by infernape

Overall Rating: 7/10