That's my team as it stands now. The Team's main goal is to give Regirock the time to set up and sweep.
Hippowdon (M) @ Leftovers
Ability: Sand Stream
EVs: 252 HP/16 Atk/152 Def/90 SDef
Impish nature (+Def, -SAtk)
- Stone Edge
- Slack Off
- Stealth Rock
- Earthquake
For Regirock, sandstorm is just plain awesome. Tyranitar shares too many weaknesses with Regi for my comfort, and so I picked Hippo. Stealth Rock can be really helpful as well. I don't really have too much more to say, other than how Hippo does a great job countering Tyranitars and Aerodactyl's. Testing this team, I don't know how successful it is against leads that use sleep moves, which is a problem.
Regirock @ Leftovers
Ability: Clear Body
EVs: 252 HP/6 Atk/252 SDef
Careful nature (+SDef, -SAtk)
- Curse
- Stone Edge
- Fire Punch
- Earthquake
Curse Regirock just plain kicks ass. With its massive Special Defense in a Sandstorm, after a Curse or two Regirock won't go down easily. I've been toying with adding either Ice Punch or Rest to the last slot, but I don't know how good of an idea that is, especially considering Earthquake's ability to rape grounded Steels.
Vaporeon (M) @ Leftovers
Ability: Water Absorb
EVs: 188 HP/252 Def/68 SpA
Bold nature (+Def, -Atk)
- Surf
- Wish
- Toxic
- Roar
As suggested, I replaced Tentacruel with Vaporeon. Vaporeon can provide Wish support for my team, which is incredibly useful for Regirock and Rotom. I'll add more after playtesting some.
Porygon2 @ Leftovers
Ability: Trace
EVs: 252 HP/120 Def/40 SAtk/96 SDef
Bold nature (+Def, -Atk)
- Discharge
- Ice Beam
- Recover
- Toxic
Counters Salamence (and other dragons) and Gyarados, and hurts a lot of potential switchins with Toxic. Bulky Waters and bulky Grounds don't like being Poisoned, and things like Blissey/Celebi can't stay in for long against Porygon if they're poisoned, especially with Sandstorm going on.
Celebi @ Leftovers
Ability: Natural Cure
EVs: 252 HP/220 Def/36 Spd
Bold nature (+Def, -Atk)
- Hidden Power [Fire]
- Leech Seed
- Grass Knot
- Recover
Celebi can switch into Earthquakes or Waterfalls and prepare to either Grass Knot something to death or just Seed and wear down opponents. Seeded enemies are also helpful for Regirock, because Regirock needs all the recovery it can get, especially considering it's reliant on Leftovers.
Rotom-h @ Choice Scarf
Ability: Levitate
EVs: 32 HP/224 Spd/254 SAtk
Modest nature (+SAtk, -Atk)
- Trick
- Thunderbolt
- Overheat
- Shadow Ball
Trick cripples walls, and Rotom is one of Trick's best users. This serves as my Scizor counter, as it resists Scizor's attacks and can threaten to Overheat. Rotom's immunites are all incredibly useful for this team, as I have two Ground and two Fighting weaknesses.
Anyway that's my team. I've been testing it, but not too much yet, so rate away.