What Cropfist said, pretty much. Even if Groudon has to go to sleep to use it, full heal via Rest makes it much tougher to bring down, especially if it can get Bulk Up beforehand.
Dunno how Karin's hax, like her Forcefield Creation & Explosion Manipulation work, but Bulk Up & Groudon likely having an AP/Dura advantage, especially with the possibility of Primal Groudon means even going up close with 4 weapons may not be enough.
It's also possible her explosions may be resisted by Primal Groudon -if Groudon can become Primal Groudon- here by way of its fire-typing.
Assuming it applies in the Cave of Origin, too....
There's also the issue that Groudon's sun would intensely heat things up, further testing her stamina, & possibly making her weapons more difficult to use if they aren't heat proof. The sun Groudon brings also amps its fire-type moves, whether it's Primal or not.
Groudon may also be able to flat out melt her weapons, thanks to moves like Eruption, Fire Blast, & Lava Plume. And it can fire instant Solar Beams thanks to its sun-summoning presence.
Add this to Earthquakes, & with Bulk Up & Rest, it can challenge her at range or, given a chance to Bulk Up, up-close. Rest's healing, too.
And Bulk Up further strengthens its Earthquakes, too!
Lastly, if BFR is considered a win condition, Groudon could possibly also achieve that through Fissure, too, assuming Karin can't fly.
I don't know Karin too well, but I'm not sure her chances are good here. My vote goes to Groudon.
