Torterra has some strong tools:
Likely attacks: Earthquake, stone edge/rock slide, wood hammer. Weaker coverage: crunch, body press, iron head, zen headbutt
Boosting etc: Curse, swords dance, shell smash, growth (for mixed sets?), reflect, light screen, iron defense
Headlong rush and superpower are possible, but it would be easy to deal with if it just spammed stat dropping moves.
Analysis: He has some really potent boosting options, and easily slots in edgequake coverage. Grass STAB is basically guaranteed, and threatens water types trying to take advantage of ground weakness to water. Stone edge/rock slide/iron head all threaten ice type attackers trying to go through the ice weakness.
Grass type attackers are likely the play here, resisting the dual stab, and not introducing a weakness to any coverage options. Shell armor invalidates strategies using crits trying to circumvent def boosts from screens/curse/iron defense/contrary shell smash. I’m not quite sure what the plan is to handle the boosting, so it depends on what they plan to do.
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Chesnaught @ CovertCloak/Shell bell/Sitrus Berry
252/252+/4/0-/0/0 (needs even HP for BD)
-Seed Bomb (Wood Hammer for OHKO strats)
-Swords Dance (Belly Drum for OHKO strats)
-Reflect
-Filler, depends on how it shakes out
Chesnaught is likely the safest attacker. He resists the dual STAB, resists edgequake, has reasonable options to hit hard with belly drum/swords dance + STAB wood hammer/seed bomb, has high phys def to work with, and learns reflect to semi-support the team. He would need support in the event of def boosting torterra. Torterra’s biggest hit is 22% max from EQ or 35.7% max from zen headbutt.
OHKO: Chesnaught kills with BD, 2 screech, HH, and atk cheer for a simple T2 OHKO. This assumes no Def boosting etc.
+6 252+ Atk Life Orb Chesnaught Helping Hand Atk Cheer Wood Hammer vs. -4 0 HP / 0 Def Tera Ground Torterra: 12613-14844 (108.8 - 128.1% of 35x hp boss) -- guaranteed OHKO
Lurantis/Serperior is another option in case we need to go from the special side. However, with the lower bulk, they will be a bit trickier to work with. Decidueye (either form) can do work with Nasty Plot, Giga Drain, Feather Dance, Frenzy Plant. Rotom-Mow (leaf storm) and Decidueye may work for OHKO strats.