Core: Palkia / Kingdra

Bring in the Ubers! This is an offensive core utilizing Palkia and Kingdra as parallel sweepers in the rain. Their typing is not the only thing they share; they have common counters, too. Palkia and Kingdra's STABs have excellent type coverage and also pack a hefty punch, allowing them to break apart teams and prepare each other for a sweep.

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Palkia @ Expert Belt

Trait: Pressure

EVs: 100 Atk / 156 SpA / 252 Spe

Hasty Nature (+Spe, -Def)

- Substitute

- Focus Punch

- Hydro Pump / Surf

- Spacial Rend

Kingdra @ Life Orb

Trait: Swift Swim

EVs: 80 Atk / 252 SpA / 176 Spe

Mild Nature (+SpA, -Def)

- Surf / Hydro Pump

- Draco Meteor

- Outrage

- Waterfall

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Common counters to Palkia and Kingdra include Dialga, Blissey, and especially Ferrothorn. It's worth noting that all three of these Pokemon are weak to Fighting-type moves. As you may have guessed, this is the reasoning behind Palkia's SubPunch set. Unfortunately, however, the only Fighting-type move Kingdra is capable of learning is Hidden Power, which is a meager 3HKO at best on Ferrothorn - not something worth implementing. For this reason, it's best to send out Palkia first to bait out your opponent's primary counter before cleaning up with Kingdra. Palkia's Expert Belt secures the OHKO on any standard Ubers Blissey (252 HP / 252 Def is very rare in Ubers due to the abundance of special attackers) with Focus Punch. Focus Punch also nets a 2HKO on both Ferrothorn and Dialga. With the opponent's primary counter down or severely weakened, Kingdra will have a significantly easier time cleaning up the rest of the opponent's team. 

In the rain, Kingdra reaches an outstanding speed stat of 500 - enough to outrun Choice Scarf Palkia. Its dual STABs allow to hit everything but Empoleon, Shedinja, and Ferrothorn at least neutrally. The combination of powerful Dragon-type moves and rain-boosted Water-type moves makes every one of Kingdra's hits hurt, even when resisted.

Kyogre is a necessary teammate for this combination to work effectively due to the permanent rain it summons. Kyogre also makes a great offensive teammate because it, too, shares counters with Kingdra and Palkia. Although it isn't necessarily recommended, if you want to get creative, Kyogre has a usable base 100 Atk stat as well as access to Brick Break as an extra Fighting-type attack.

With the same typing comes the same weaknesses - even if it is only one. Both Kingdra and Palkia are weak to Dragon-type attacks, so using at least two Steel-type Pokemon on your team is advised. Also, entry hazards makes a sweeper's job significantly easier. Ferrothorn makes a good teammate for its many resists, including Dragon, and its ability to set up Spikes and force switches with Leech Seed. Jirachi makes an acceptable partner because of its ability to use Stealth Rock and pass Wishes to allow multiple attempts at sweeping. Skarmory can also set up entry hazards and makes a great check to Bulk Up Dialga, possibly the most threatening Dialga set to this core due to the fact that by alternating between Rest/Sleep Talk and Bulk Up, it can set up against even Focus Punch Palkia. Steel-types in general make great partners also because their Fire-type weakness is at least partially negated in the rain, making them more durable.

Palkia and Kingdra are two of the most threatening Pokemon in Ubers under the right conditions, and with their counters weakened or down, they are nearly unstoppable as a duo.

Damage Calcs:

Palkia Focus Punch vs 252 HP Dialga: 63.37% - 75.25% 2HKO

Palkia Focus Punch vs 252 HP / 4 Def Calm Blissey: 190.2% - 224.09% RAPE

Palkia Focus Punch vs 252 HP / 252 Def Bold Blissey: 83.61% - 98.46% 66% OHKO*
Palkia Focus Punch vs 252 HP / 4 Def Ferrothorn: 73.58% - 87.22% 2HKO

*with SR in play.