Well I played 35 ladder games and a few tours with Groudon spam, so I have a lot of experience with these teams. Here are some things that I've noticed that defeat it:
1) Two shedinjas. Sturdy + Scrappy or two Sturdies; Groudon Spam has a 99.9% chance of losing to this team assuming you know what you're doing. Most of the time Groudon spams users might just forfeit at team preview.
2) One well-played shedinja. Groudon teams can't run stealth rock, and Shed switches into every single Groudon-P barring trick + flame orb or Mold breaker. Chansey can't beat it 1v1 (especially if you bring something like night shade), and all you need to do is predict the mold breaker groudon-P by baton passing out and into your protean or something. Oh yeah, and if it goes for the RK, so be it. Your opponent is losing pokemon at a faster rate than you are. Overall the least gimmicky answer to Groudon-P.
3) Mega Slowbro with Surf and something that makes it imposterproof. Groudon-P literally can't touch 2HKO this thing, and hell, you don't even need fur coat. Another very reliable answer to Groudon that wins 75% of the time.
4) Fur Coat. This one has been discussed and is currently one of the more effective options. However, I don't really think things like Cresselia should be using Fur Coat to beat this. Offensive Fur Coat fares far, far, far better than defensive fur coat.
5) Any physically bulky resist, such as Tangela/Tangrowth, Mega Heracross,Surskit, or Gourgeist-XL. Assuming the set doesn't get wrecked by imposters, you can't really lose with one of these on your side as long as you know how to look for tinted lens. For example, Groudon-P's Banded 1k Arrows doesn't even 3HKO Mega Heracross when fully invested.
6) PH Coil sweepers that aren't weak to Ground and have decent bulk (maybe coil + will-o + 1k Arrows + KS regular Groudon)
7) Assist Kyo-P teams
Also here are my thoughts on prankster taunt as a solution:
I see this mentioned so much, and the thing is, it's SO bad against these teams. They can literally switch in their bouncer (everyone assist team has one) and proceed to force you to switch out. Or if they're using their prankster one, it can simply phase you out. Or let's say you do manage to taunt it. Well guess what; as you parting shot out, trying to steal momentum, they know your whole set now, and can now switch in their magic bouncer on you. Unless you run something like scald PrimOgre (which is really one of, if not the only prankster taunt mon that beats it), Prankster Taunt becomes almost dead weight against this team. Even if you run PrimOgre, scald forces too many guessing games for it to actually be considered a solid check to Groudon Spam. Please, if you're running this for the sole purpose of beating groudon-spam...don't.
1) Two shedinjas. Sturdy + Scrappy or two Sturdies; Groudon Spam has a 99.9% chance of losing to this team assuming you know what you're doing. Most of the time Groudon spams users might just forfeit at team preview.
2) One well-played shedinja. Groudon teams can't run stealth rock, and Shed switches into every single Groudon-P barring trick + flame orb or Mold breaker. Chansey can't beat it 1v1 (especially if you bring something like night shade), and all you need to do is predict the mold breaker groudon-P by baton passing out and into your protean or something. Oh yeah, and if it goes for the RK, so be it. Your opponent is losing pokemon at a faster rate than you are. Overall the least gimmicky answer to Groudon-P.
3) Mega Slowbro with Surf and something that makes it imposterproof. Groudon-P literally can't touch 2HKO this thing, and hell, you don't even need fur coat. Another very reliable answer to Groudon that wins 75% of the time.
4) Fur Coat. This one has been discussed and is currently one of the more effective options. However, I don't really think things like Cresselia should be using Fur Coat to beat this. Offensive Fur Coat fares far, far, far better than defensive fur coat.
5) Any physically bulky resist, such as Tangela/Tangrowth, Mega Heracross,
6) PH Coil sweepers that aren't weak to Ground and have decent bulk (maybe coil + will-o + 1k Arrows + KS regular Groudon)
7) Assist Kyo-P teams

Also here are my thoughts on prankster taunt as a solution:
I see this mentioned so much, and the thing is, it's SO bad against these teams. They can literally switch in their bouncer (everyone assist team has one) and proceed to force you to switch out. Or if they're using their prankster one, it can simply phase you out. Or let's say you do manage to taunt it. Well guess what; as you parting shot out, trying to steal momentum, they know your whole set now, and can now switch in their magic bouncer on you. Unless you run something like scald PrimOgre (which is really one of, if not the only prankster taunt mon that beats it), Prankster Taunt becomes almost dead weight against this team. Even if you run PrimOgre, scald forces too many guessing games for it to actually be considered a solid check to Groudon Spam. Please, if you're running this for the sole purpose of beating groudon-spam...don't.