Okay, so I completely forgot about my Primal Groudon reservation here. :P Apologies
Primal Groudon
EVs: HP: 252; SpD: 252; Speed: 4
Nature: Careful (+SpD, -SpA)
Moves:
Dragon Claw
Toxic
Rock Tomb
Precipice Blades
Hello all! So, the logic here is that Primal Groudon can be used to contend with a few of the threats that Poisonceus might face. There are some weaknesses, but I hope to address them and hopefully account for some of them with the partnership with Pdon. :)
To start out with, some weaknesses...
252+ SpA Life Orb Deoxys-Attack Psycho Boost vs. 252 HP / 252 SpD Groudon-Primal in Psychic Terrain: 530-625 (131.1 - 154.7%) -- guaranteed OHKO ---> Yeah, so unfortunately, that is still a thing. :P Even with full SpD and HP investment, Primal Groudon cannot survive a Psycho Boost hit under Psychic Terrain, and certainly Poisonceus cannot either. Even without Psychic Terrain up, it is dicey...
252+ SpA Life Orb Deoxys-Attack Psycho Boost vs. 252 HP / 252 SpD Groudon-Primal: 352-417 (87.1 - 103.2%) -- 18.8% chance to OHKO
However, that is a threat that can hopefully be remedied by other components to the team.
Should Primal Kyogre switch into Pdon, cancelling out Desolate Land, that could pose an issue: 252+ SpA Kyogre-Primal Water Spout (150 BP) vs. 248 HP / 148 SpD Arceus-Poison in Heavy Rain: 390-459 (88 - 103.6%) -- 25% chance to OHKO
The good news is that provided you get the switching down right, there is little that Primal Kyogre can do to Primal Groudon, as Ice Beam will do somewhat marginal damage: 252+ SpA Kyogre-Primal Ice Beam vs. 252 HP / 252+ SpD Groudon-Primal: 107-126 (26.4 - 31.1%) -- guaranteed 4HKO
Versus this...
0 Atk Groudon-Primal Precipice Blades vs. 0 HP / 4 Def Kyogre-Primal: 235-277 (68.9 - 81.2%) -- guaranteed 2HKO
Now, for some better news...
0 Atk Groudon-Primal Precipice Blades vs. 0 HP / 0 Def Groudon-Primal: 288-342 (84.4 - 100.2%) -- 6.3% chance to OHKO ---> The good news with this Primal Groudon set I am recommending for partnership is it can fight with other Primal Groudon forms. Even with no attack investment, should it come across a more offensive Pdon set, it can stand its ground and fight back with Precipice Blades to try and eliminate that threat for Poisonceus.
This set I feel is formidable against Ho-Oh with the combination of Rock Tomb, Toxic, and Stealth Rock from Poisonceus. It can wear it down quickly, which can help Poisonceus since it wouldn't be able to do a ton against Ho-Oh.
With Rock Tomb at Pdon's disposal, Dragon Claw can become helpful to deal with Ultra and Mray.
252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 252 HP / 0 Def Groudon-Primal: 238-281 (58.9 - 69.5%) -- guaranteed 2HKO
0 Atk Groudon-Primal Dragon Claw vs. -1 0 HP / 4 Def Rayquaza-Mega: 288-340 (82 - 96.8%) -- guaranteed 2HKO
252 Atk Necrozma-Ultra Earthquake vs. 252 HP / 0 Def Groudon-Primal: 176-208 (43.5 - 51.4%) -- 6.6% chance to 2HKO
0 Atk Groudon-Primal Dragon Claw vs. 0 HP / 0 Def Prism Armor Necrozma-Ultra: 148-175 (44.1 - 52.2%) -- 16% chance to 2HKO
Summary:
Overall, a partnership between Primal Groudon and Poisonceus is not perfect, but I believe it can check some common threats in AG and be formidable provided proper playing and a good support team around the duo. :)