I think the over-the-top aspect of primal Groudon that makes him so ridiculously threatening is his immunity to both burn and t-wave. It may seem like an obvious point at first, but think about it - Arceus/Giratina willowisps and prankster t-waves have always been an almost 100% check to every set up sweeper out there: ekiller, xerneas, anything. Not to mention the random scald burns and sacred fires sprinkled in. Sure, you may end up sacrificing a 'mon to get that crucial status off but afterwards you can consider it a neutralized threat.
Groudon completely throws this out the window though. In terms of status, all you can do is toxic and wait it out, which is a pretty horrible strategy to cling to when he proceeds to wipe through 2 or 3 of your pokemons before succumbing to the poison and weak priority moves (100/160 phys defense remember, lol). So, anything that can't really threaten groudon with a really strong special attack or a ground stab is basically free set up, even if they get off a toxic or whatever. (And don't forget that toxic misses)
I think that paralysis is a somewhat underutilized status effect, and the obvious speed drop but more subtly the parahax chances, are extremely useful in ubers. In a meta where everything is advised to use missable moves like thunder and focus blast and fire blast, having that 25% hax chance can do all kinds of things in your games and skew probabilities. (I recently read a post that had carefully calculated the probabilties for parafusion to beat a chansey, lol) But obviously, in this particular case, it's more about the permanent speed loss, which can allow something fairly slow but strong on the special side to finish off Groudon.
To that end, I wanted to put forward the idea of ways to paralyze groudon despite his ground typing. That basically only leaves Glare, Stun Spore, and Body Slam as options, and it's pretty slim pickings.
So, although there are reliable options like Gyarados and Rayquaza for dealing with a boosted Groudon, there may be a scarce handful of other pokemon out there that can somewhat check groudon in their own way, like Whimsicott and Altaria.
That was a pretty longwinded post just to outline the possibilities of getting status on Groudon, but, that's really the way that I tend to play pokemon anyway and whether defensive playstyles will be a viable option or not remains to be seen. Hopefully all the offense players out there will do their best to force most groudons to only carry 1 boosting move instead of both :)