Registeel @ Reginite  
Ability: Clear Body  
Level: 50  
EVs: 252 HP / 4 Atk / 132 Def / 116 SpD / 4 Spe  
Impish Nature  
- Heavy Slam  
- Iron Defense  
- Body Press  
- Protect

Unown
Move options:
- supportive: Amnesia, Thunder Wave

Unown
Offensive calcs:
- +2 132+ Def Mega Registeel Body Press vs. 236 HP / 4 Def Incineroar: 258-304 (129 - 152%) -- guaranteed OHKO
Defensive calcs:
- 252+ SpA Torkoal Eruption (150 BP) vs. 252 HP / 116 SpD Heatproof Mega Registeel in Sun: 68-84 (36.3 - 44.9%) -- guaranteed 3HKO
- 252+ SpA Mega Charizard-Y Heat Wave vs. 252 HP / 116 SpD Heatproof Mega Registeel in Sun: 66-80 (35.2 - 42.7%) -- guaranteed 3HKO
- 252+ Atk Landorus-T Earthquake vs. 252 HP / 132+ Def Mega Registeel: 78-92 (41.7 - 49.1%) -- guaranteed 3HKO
- 252 SpA Life Orb Sheer Force Landorus Earth Power vs. 252 HP / 116 SpD Mega Registeel: 133-159 (71.1 - 85%) -- guaranteed 2HKO

Unown
Useful partners:
- healing / redirection: Comfey, Tapu Fini, Blastoise, Clefairy, Amoonguss
- any offensive Pokémon complementing its Steel type: Water, Fairy, Flying, etc.