Hey rileydelete, great to see you getting back into the game, we've missed you!
A core that I have invented specifically in an attempt to address the issue you have raised consists of the following: Skarmory, Ho-Oh, and Arceus-Ground. Spreads and sets are unimportant for the most part with the only nuance being that Arceus-Ground must be carrying Judgment, so something along the lines of Judgment, Ice Beam/Defog, Toxic/Defog, and Recover would work.
The first step in creating a core that counters a Pokémon as over-centralizing as Primal Groudon is to identify all of its viable sets, which consist of a dichotmy between offensive with Precipice Blades and Stone Edge, offensive with Fire Blast, and supportive with Stone Edge, and supportive with Lava Plume. In our case, Arceus-Ground counters the former two, Skarmory the support with a Rock-type move, and Ho-Oh the support with a Fire-type move.
I have taken a liking to teaching both Skarmory and Arceus-Ground Defog to support Ho-Oh's greatest weakness to the maximum. The crux of this core is to bait the Primal Groudon into revealing its moveset as you absorb incoming hits with respective appropriate switch-ins until you identify which of the three is the full-on counter to the specific Primal Groudon you are facing. The only scenario unexamined thus far is that of a supportive set sporting both a Rock and Fire-type move, but these are rarely encoutered in higher level play as supportive Primal Groudon is always equipped with Precipice Blades, a Xerneas solution, and Stealth Rocks; if worst comes to worst, Arceus-Ground is there to kill it outright.
The only forseeable scenario in which this core is broken would be by a Primal Groudon carrying both Eruption and Stone Edge in combination with either Trick Room, Sticky Webs, or Tailwind, but these can be counter-played/played around with ease with the correct sequences thanks to the bulk of the core and access to Defog.
A team that I personally use utilizing this exact core consists of the following: Primal Groudon, Skarmory, Mega-Salamence, Xerneas, Ho-Oh, and Arceus-Ground, and the following is a replay of me using a variation of the team with Arceus-Ground's role delineated into Mega-Gengar and Arceus Water:
http://replay.pokemonshowdown.com/ubers-376690411
I think the replay accurately reflects upon how the core works together flexibly to ensure Primal Groudon is rendered non-threatening.
Good luck man, it's awesome to see you swinging around these corners of the Internet! xD
-Henri