Ok so let's try this. The first Pokemon that came to mind is Lickilicky!
I don't know why.
Lickilicky @ Leftovers
Ability: Oblivious
EVs: 252 HP / 56 Atk Def / 200 SpD
Careful Nature
- Body Slam/Return
- Rock Tomb/Rock Slide
- Knock Off
- Filler
A few calcs...
56 Atk Lickilicky Return vs. 0 HP / 4 Def Thundurus: 72-85 (46.7 - 55.1%) -- 70.7% chance to 2HKO
56 Atk Lickilicky Body Slam vs. 0 HP / 4 Def Thundurus: 60-72 (38.9 - 46.7%) -- guaranteed 3HKO
56 Atk Lickilicky Rock Slide vs. 0 HP / 4 Def Thundurus: 72-86 (46.7 - 55.8%) -- 73.4% chance to 2HKO
56 Atk Lickilicky Rock Tomb vs. 0 HP / 4 Def Thundurus: 56-68 (36.3 - 44.1%) -- guaranteed 3HKO
252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 200+ SpD Lickilicky: 75-91 (34.5 - 41.9%) -- 84.4% chance to 3HKO after Leftovers recovery
252 SpA Life Orb Thundurus Focus Blast vs. 252 HP / 200+ SpD Lickilicky: 135-161 (62.2 - 74.1%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Life Orb Thundurus Hidden Power Ice vs. 252 HP / 200+ SpD Lickilicky: 35-42 (16.1 - 19.3%) -- possible 8HKO after Leftovers recovery
Oblivious is a big draw in Licki's favour although with SwagPlay an option you certainly cannot dismiss Own Tempo. While Thundy will of course pose a big threat back, support is viable, SR, Roar, EQ if under Gravity or Smack'd Down
56 Atk Lickilicky Rock Tomb vs. 0 HP / 4 Def Thundurus: 56-68 (36.3 - 44.1%) -- 97.3% chance to 2HKO after Stealth Rock
56 Atk Lickilicky Earthquake vs. 0 HP / 4 Def Thundurus: 94-112 (61 - 72.7%) -- guaranteed 2HKO after Stealth Rock
Playing with the Attack EV's...
252 Atk Lickilicky Earthquake vs. 0 HP / 4 Def Thundurus: 114-136 (74 - 88.3%) -- 93.8% chance to OHKO after Stealth Rock
252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 4+ SpD Lickilicky: 94-110 (43.3 - 50.6%) -- guaranteed 3HKO after Leftovers recovery
And that's just with the typing advantage. It turns Return and Body Slam into guaranteed 2hko, doing over 50%.
I find it important enough to edit that Rollout is probably the best move to use. I'll need to revisit the calculator as when I looked initially, it wouldn't let me do anything. If Licki can set up, it may only need one boost for a 2HKO.