Primal Groudon is arguably the most powerful pokémon in the entire game. Ground/Fire is a tremendous type combination which is only weak to Ground thanks to Desolate Land. And Earth Power isn't that widely spread, while the other Ground moves are physical and Groudon can easily take those with its tremendous physical bulk. Base 180 attack combined with Swords Dance access makes sure it hits very very hard. With the right support in doubles, there is nothing that scares Primal Groudon.
Groudon @ Red Orb
Ability: Drought
EVs: 4 HP, 252 Attack, 252 Sp.Def
Nature: Brave (get as low speed IV as possible, ideally 0 or 1)
- Precipice Blades
- Fire Punch
- Swords Dance
- Protect
I've been running this with Trick Room support. Ideally you use a very slow Trick Room setter which can also use Skill Swap and Gravity. I personally like Stakataka for the role. Skill Swap resets Desolate Land to remove the weathers caused by Primal Kyogre and Mega Rayquaza, and can give Groudon a new ability. Stakataka gives it Beast Boost, which Groudon doesn't really need that much (Precipice Blades tends to destroy teams already at +2 without needing more power) but is still terrifying and comes into play more if you didn't manage to get Swords Dance up. Gravity is Gravity on the partner, ideal to boost Precipice Blades's accuracy so you won't get those unlucky misses anymore, and it also pulls down all those annoying Levitators and Flying types like Yveltal, Mega Rayquaza, and Ho-Oh to be destroyed by Precipice Blades. Primal Groudon's STAB combination is completely unresisted in Gravity conditions outside of Water Bubble Araquanid (and Dewpider). Precipice Blades is the move to spam. A lot of power, almost unresisted with Gravity support, STAB, hits both opponents, and perfectly accurate in Gravity. It is terrifying. Fire Punch is a way to inflict damage when the enemy has Wide Guard and hits the occasional Grass or Bug Type for super effective damage. Swords Dance is the boosting move, set it up while the partner sets up Trick Room, Groudon is bulky enough to get away with it most of the times, and Protect is primarily filler but still useful from time to time in doubles.
The EVs are quite straightforward. Max attack to hit as hard as possible, and max Sp.Def to get extra bulk against special attacks. Max Sp.Def allows Groudon to survive some heavy special attacks better than a max HP investment would give you (like some Z-Moves, Adamant Orb Dialga's Roar of Time, and Specs Kyurem-White's Ice Beam for instance). Groudon doesn't need more bulk in its physical defence with its base 160 defence stat.