Bold works. You can also do Sassy or Relaxed. Modest works too. But i see you want to run it specially. So don’t do Impish at ALL.
D. Registeel @ Leftovers/Petaya Berry
Ability: Flame Body (Foundry if possible)
EVs: 252 HP / 252 SpA / 4 Def
Modest/Sassy/Bold/Quiet Nature
- Lava Plume/Ancient Power (IF FOUNDRY)
- Recover
- Stealth Rock
- Focus Miss/Thunderbolt
But honestly? It has better options on the physical side.
D. Registeel @ Leftovers/Petaya Berry
Ability: Flame Body (Foundry if possible)
EVs: 252 HP / 252 Atk / 4 SpD
Adamant/Sassy/Brave Nature
- Fire Punch or Stone Edge/Rock Slide (IF FOUNDRY)
- Recover
- Stealth Rock
- Hammer Arm/Thunder Punch
Pick your poison. Or, if you eat in the school cafeteria, pick your mystery meat.