Regirock @ Rocky Helmet
Trait: Clear Body
EVs: 252 HP / 252 Atk / 4 SDef
Impish Nature
- Drain Punch
- Earthquake
- Stealth Rock
- Stone Edge
Musharna (M) @ Leftovers
Trait: Synchronize
EVs: 240 HP / 252 Def / 16 SAtk
Bold Nature
- Calm Mind
- Heal Bell
- Psychic
- Moonlight
Regirock takes care of common physical threats in the tier while taking resisted attacks and somewhat healing from drain punch. Any special attacker can be countered with a quick switch to musharna. A quick CM or nice switch could swap the games momentum in your favor. A threat to this core would be Ludicolo but 1 of the 4 other spots on your team could be filled with a pokemon to counter Ludicolo.