Hydrapple @ Tera Shard  
Ability: Regenerator  
Tera Type: Dragon  
EVs: 248 HP / 252 Def / 8 SpA  
Relaxed Nature  
IVs: 0 Spe  
- Dragon Rage  
- Earth Power  
- Shelter  
- Rollout

Hydrapple is another good check as with good bulk, resistance to thundebolt, and the ability to just take advantage of it and pivot out to a teammate. This gives it good presence against Thundurus as it makes it harder for it to force progress against it. Although it has to be careful of focus blast and just being worn down over the course of a game as with hazards, thundy-i solid damage output, and other potential teammates of theirs it can make it difficult to actually stay at high health