Before you say, 'hey 2x, that Whimsicott should be using Infiltrator!', remember: Prankster gives Whimsicott's Nature Power priority.
This is a very straightforward, easy-to-use team that addresses as large a portion of the meta's threats as possible. Regirock, Talonflame, and AV Guzzlord form the team's main defensive backbone, with Regirock and Talonflame handling hazard setting and removal, respectively. Iron Defense Body Press Regirock is pretty busted as a late-game wincon, but otherwise it's a very effective SR user in a meta where our best Defog user is Talonflame. It also just blanket checks many Pokemon you think it wouldn't be able to normally, especially once it's boosted its already massive Defense stat.
Talonflame is run here with just enough Speed for opposing Whimsicott, so that the remainder of its EVs can be allocated toward its Defense in order to better answer the meta's Fighting-types. Between Toxicroak, Whimsicott, and Talonflame, Fighting-types like Virizion are pretty well-covered. Just be wary of Knock Off, and don't be afraid to call it out and switch to something other than Talonflame early-game.
With Talonflame and Regirock being relied on heavily here, Toxicroak was added to cover both of them against Water-types, particularly Vaporeon, and uses those as setup fodder via Nasty Plot. In this meta, NP is a stronger set than SD, and this team can kind of struggle if you're unable to position yourself well against threats like Weezing, Palossand, and Sandaconda (basically the Pokemon that prevent Togedemaru from cleaning up late-game), so NP Toxicroak was an easy fit for this team.
Next, even despite Whimsicott's base 77 SpA stat, its Specs set really slaps in this meta. In a way, Specs Whimsicott is taking on two roles for the team by itself: (1) speed control (via priority Nature Power, which is coincidentally useful against people trying to cheese you via terrain-based teams - Nature Power becomes priority Energy Ball on Grassy Terrain and priority Thunderbolt on Electric Terrain) and (2) wallbreaker. The resists it provides are also useful, but it is flimsy enough that it shouldn't be played defensively if it can be helped. Hurricane is a nice midground move, and hits all of those Pokemon that might try to pivot into Energy Ball pretty hard; for example, it happens to 2HKO Weezing:
252 SpA Choice Specs Whimsicott Hurricane vs. 252 HP / 0 SpD Weezing: 170-200 (50.8 - 59.8%) -- 89.1% chance to 2HKO after Black Sludge recovery
Alternatively, Psychic is an option here over Hurricane, but do not remove Nature Power! The status it provides and the ability to dodge Sucker Punch while still attacking will clutch out a lot of games. Basically, locking into Nature Power or Hurricane will put a lot of pressure on Whimsicott switch-ins like Talonflame early- to mid-game, so try to focus on using Whimsicott offensively.
Setting up with Regirock and Toxicroak or breaking with Whimsicott isn't always free though, so AV Guzzlord was added for offensive glue and additional breaking power, more or less, with that shoring up the team's weakness to Fire-types like Magmortar and Charizard, as well as other strong special attackers like Heliolisk. Additionally, it adds an important Ghost resist (which many people forget in this meta) and gives you an out against Poltergeist users that aren't named Froslass and Ghost-types like Rotom and Palossand.
Lastly, I was looking for some solid speed control in addition to Toxicroak's Vacuum Wave and Whimsicott's Nature Power, as well as a strong check to Pokemon like Arctovish (even when its Slush Rush is active), opposing Whimsicott, and Ribombee, so Choice Scarf Togedemaru fit the bill nicely. Togedemaru brings in a lot of passive Pokemon via its U-turn, too, which in turn allows Toxicroak to set up NP and Whimsicott and Guzzlord to wallbreak.
Just to check that this team was user friendly, I passed this to
tlenit, who tested this for me on ladder. Anyway, below are some replays in case you want to know more about how this team should be used. Enjoy!
https://replay.pokemonshowdown.com/gen8pu-1276307419-8h0wijs9c08jgrepdyrzrgtk3sq8j7npw tlenit vs kink
https://replay.pokemonshowdown.com/gen8pu-1276038616-fqhpy1i0hb4ygzbar2k9skh69c3yj3vpw Specs Whimsicott and NP Toxicroak breaking against Chloe's sample
https://replay.pokemonshowdown.com/gen8pu-1276032045-qndqlkz1t8k2jj7h70w20srw9der7oapw priority Nature Power coming through
https://replay.pokemonshowdown.com/gen8pu-1276042852-gswwou9h2hsbr7k9jurnmkbm62pudpapw
https://replay.pokemonshowdown.com/gen8pu-1276078788-lj3m5bntaazvsf9vk5nn8dagltczam2pw
https://replay.pokemonshowdown.com/gen8pu-1276023736-6qsk64x38f5ni9xoynj34v6ntzyi92rpw
https://replay.pokemonshowdown.com/gen8pu-1276019337-ybxcwih6tkrozl6ohdwcksdz6ajkdx4pw
https://replay.pokemonshowdown.com/gen8pu-1276019925-bgx6zf46qr6c6zi5jq1btl853q74odzpw