To be honest, Groudon doesn’t need a partner with a sun-based ability, as Chlorophyll Pokémon are grass types, which don’t need to hit water types because of Desolate Land, and Solar Power Pokémon, which are almost all fire types, and you don’t need more fire coverage. The chlorophyll Pokémon I could see working are:
Chlorophyll After you + Eruption Shenanigans and Fast Sleep Powder.
Rage Powder and Sleep Powder support.
A decent sweeper on its own, but you can also entrainment Chlorophyll onto another Pokémon for more shenanigans.
Now, these are some other Pokémon that don’t really benefit from sun but are still great partners to Groudon.
One of the most common duos back in VGC 2016, when Primal Groudon was first released. Basically just exerts a ton of offensive pressure. Groudon also covers steel and ground types for Xerneas.
- Mega Salamence
Salamence is another hyper-offensive pairing, plus it’s immune to earthquake and can set Tailwind, as Groudon is Mid-Speed Tier. Other flying Types with Tailwind would also be good.
- Mega Gengar
Not as useful in-game, but back in 2016, weather wars were big, so trapping Kyogre with Shadow Tag made Groudon not threatened at all by it.
Hope this helps :)