Check

Okidogi @ Heavy-Duty Boots
Ability: Toxic Chain
Tera Type: Ground
EVs: 252 Atk / 240 SpD / 16 Spe
Jolly Nature
- Drain Punch
- High Horsepower
- Toxic
- Poison Jab
Okidogi can resist every single move that darkrai has on this set and threaten it back with a drain punch, or a high horserpower if it tera's. Toxic allows okidogi to reliably get something off on darkrai unless it tera poison's on that exact turn, while posion jab is just good stab. However, at +2, dark pulse can 2hit ko okidogi, so it does have to be careful.
+2 252 SpA Life Orb Darkrai Dark Pulse vs. 0 HP / 240 SpD Okidogi: 153-182 (48.2 - 57.4%) -- 48.8% chance to 2HKO after Leftovers recovery
The ev's are max attack to deal as much damage as possible, the jolly and speed ev's allows okidogi to outspeed jolly gambit and the rest go into special defense to live as many hits as possible. Tera ground boosts up high horsepower's damage as it sadly doesn't ko from full.
252 Atk Okidogi High Horsepower vs. 0 HP / 0 Def Tera Poison Darkrai: 226-266 (80.4 - 94.6%) -- guaranteed 2HKO
Counter

Clodsire @ Heavy-Duty Boots
Ability: Unaware
Tera Type: Bug/Fairy
EVs: 248 HP / 8 Atk / 252 SpD
Careful Nature
- Megahorn
- Amnesia
- Recover
- Earthquake
8 Atk Clodsire Megahorn vs. 0 HP / 0 Def Darkrai: 150-178 (53.3 - 63.3%) -- guaranteed 2HKO
Yeah, clodsire gets megahorn, who would've thunk (probably due to the protuding poison spikes it has on its back when attacking in-game).
This clodsire set can deal massive damage to darkrai while not taking much in return, as dark pulse does max 32% back to it. This allows clodsire to get off an amnesia and recover almost all of the time. Then it can threaten darkrai back with a megahorn as clodsire has enough longevity and raw bulk to afford the misses from megahorn. If darkrai tries to tera poison to avoid being ko'd by megahorn, then e-quake is going to finish the job off even quicker.
Ev spread is standard and the tera types are customisable, you can choose more than the ones provided. Tera bug allows megahorn to deal even more damage, which is just funny, while tera fairy resists both dark pulse and focus blast allowing you to get off an amnesia before they can hit a sludge bomb.
Some calcs for the funnys:
8 Atk Tera Bug Clodsire Megahorn vs. 0 HP / 4 Def Rillaboom: 224-266 (65.6 - 78%) -- guaranteed 2HKO after Grassy Terrain recovery
252+ Atk Choice Band Rillaboom Wood Hammer vs. 248 HP / 0 Def Tera Bug Clodsire in Grassy Terrain: 308-363 (66.5 - 78.4%) -- guaranteed 2HKO after Leftovers recovery and Grassy Terrain recovery
8 Atk Tera Bug Clodsire Megahorn vs. 0 HP / 0 Def Ogerpon-Wellspring: 236-282 (78.4 - 93.6%) -- guaranteed 2HKO
8 Atk Tera Bug Clodsire Megahorn vs. 0 HP / 0 Def Roaring Moon: 272-324 (77.4 - 92.3%) -- guaranteed 2HKO
8 Atk Tera Bug Clodsire Megahorn vs. 0 HP / 0 Def Weavile: 294-348 (104.6 - 123.8%) -- guaranteed OHKO
252 Atk Weavile Triple Axel (120 BP) (3 hits) vs. 248 HP / 0 Def Tera Bug Clodsire: 283-335 (61.1 - 72.3%) -- approx. 2HKO after Leftovers recovery
8 Atk Tera Bug Clodsire Megahorn vs. 0 HP / 0 Def Samurott-Hisui: 248-294 (77.2 - 91.5%) -- guaranteed 2HKO