Primal-Groudon is definitely one of the hardest Pokémon to switch into, along with Mewtwo, Mega-Rayquaza, Primal-Kyogre, Necrozma-Dusk-Mane, and Arceus-Ground. While most people just use SpDef Rock's, Primal-Groudon has a lot of viable sets. Rock Tomb + Eruption eats most Pokémon alive. Anything that resists would hate being Toxic'ed. Mixed RP is a fantastic late-game sweeper for balance teams. Primal-Groudon is terrible to beat.
Well, the primary way of beating Primal-Groudon is by determining it's set. You generally beat it through Toxic or pressuring teammates with heavy hitters that force Primal-Groudon, like Primal-Kyogre and Mega-Rayquaza. If you somehow know your opponent is going to have mixed RP Primal-Groudon, Mantine is probably one of the best answers.
Mantine @ Leftovers
Ability: Water Absorb
EVs: 240 HP / 16 Def / 252 SpD
Calm Nature
IVs: 0 Atk
- Roost
- Haze
- Toxic
- Scald / Defog
Lovely typing, fat SpDef, and a radical movepool. AG Mantine is essentially a weird combination of defensive Ho-Oh (on the SpDef side), Gastrodon
(#SlugForDRank s/o
Corpse), and Goggles Zygarde-C. Mantine can essentially check any special sweeper that isn't carrying super-effective coverage (seriously! It has a chance to eat special Light Burn into Photon Geyser. It WALLS non-Thunder Ogre (which it could form a nice duo with SpDef Primal-Groudon).
The primary Mantine moveset consists of Toxic, Haze, Roost, and Defog/Scald, but you could also try Ice Beam, Icy Wind, Mirror Coat, Mud Slap, Sleep Talk, Tailwind,
Water Sport(edit: that is not Water Spout), or Whirlpool.
With regards to our friend Primal-Groudon, here's some calcs:
252+ SpA Groudon-Primal Fire Blast vs. 240 HP / 252+ SpD Mantine in Harsh Sunshine: 93-111 (25 - 29.9%) -- guaranteed 4HKO
252+ SpA Groudon-Primal Dragon Pulse vs. 240 HP / 252+ SpD Mantine: 65-77 (17.5 - 20.7%) -- possible 5HKO
Groudon-Primal Precipice Blades vs. Mantine: 0-0 (0 - 0%) -- possibly the worst move ever