All-out attacker Palkia is gifted with the right amount of power and coverage to be able to 2HKO nearly the entire metagame without having to resort to a Choice Specs set. This makes it extremely effective at tearing apart defensive cores as it requires little to no prediction. The freedom to change attacks allows it to maintain momentum and avoid becoming setup fodder. The versatility this set offers also helps to make it unpredictable, further complicating the process of checking it.
Spacial Rend is a reliable Dragon-type STAB and its ability to easily topple specially defensive Dragon-types like Giratina is what separates Palkia from other powerful Water-type special attackers. STAB Surf complements Spacial Rend excellently and is absurdly powerful under the rain, capable of 2HKOing bulky Arceus formes. Hydro Pump can be used in its place to secure more OHKOs than Surf; however, the inaccuracy is off-putting, and a rain-boosted Surf already 2HKOes everything you would use Hydro Pump for. Fire Blast is extremely important for 2HKOing Ferrothorn under rain and punishes Arceus-Grass outside of rain. Thunder is a solid option for the last slot as it allows Palkia to fully fill its common role of checking Kyogre. However, Dragon Tail is very useful for preventing Calm Mind Arceus formes from setting up on Palkia and, with entry hazard support, is effective at wearing down Palkia's few checks, making it the superior choice for teams that aren't quite as concerned with Kyogre. Keep in mind that specially defensive Kyogre becomes quite a nuisance to Palkia if it does decide to pass on Thunder.
Lustrous Orb allows Palkia to OHKO Forretress with Surf after Stealth Rock as well as give a significant boost to Palkia's main STABs for general purposes. As Palkia's sole weakness is to Dragon-type attacks, Haban Berry allows Palkia to be an effective lure although at the price of power. Leftovers is another item choice that helps Palkia check Choice Scarf and defensive Kyogre more effectively; however, it misses out on the same power Haban Berry does. For those that dislike paralysis or burn from Kyogre's Thunder or Scald, Lum Berry is useful to keep Palkia in good condition. However, this is a one-time solution to a side effect of an already painful Thunder or a surely recurring burn from the long-lasting defensive Kyogre, making the loss in damage output all the more bothersome. Lastly, an Expert Belt can be used for those who would like to favor Palkia's super effective coverage options; however, its main STABs lose their consistent boost in power.
Palkia can use Aqua Tail with a Hasty nature over Thunder and Dragon Tail to 2HKO Chansey. Focus Punch can also be used with a Hasty nature over Fire Blast to hit Chansey while retaining Thunder or Dragon Tail and not losing automatically to Ferrothorn. Substitute can be used with or without Focus Punch to punish switches and protect Palkia from status; however, the loss in coverage generally increases the number of checks to Palkia. For teams that struggle against Ho-Oh in the sun, Stone Edge can be used to deal significant damage to it. Outrage can be used with Haban Berry to OHKO Latias; however, the result of getting locked into Outrage makes this option mediocre at best.
As dangerous and effective as Palkia is, it still appreciates team support to be at its best. Having a strong response to Chansey and Arceus-Grass, especially for rain teams, is worthwhile. Fortunately, these two are quite easy to take advantage of, leaving you with multiple options. Forretress, Skarmory, and Ferrothorn are generally safe switch-ins to both Chansey and Arceus-Grass and can set up Spikes or use Rapid Spin without much risk. Watch out for Calm Mind or Fire Blast Arceus-Grass variants that might set up or KO your Spikes setter however. Tornadus-T and Ho-Oh are good offensive options to deal with Palkia's two main counters, but they need to be careful of switching into Toxic or Ice Beam in the case of Tornadus-T. Genesect and Kyurem-W are also good answers to Arceus-Grass, although the former needs to watch out for Will-O-Wisp and Fire Blast. Also, keep in mind that Arceus-Grass can be built to give Genesect a Special Attack boost, making it difficult to take down with U-turn when hazards are up. Powerful Fighting-types, such as Terrakion, can be used to force Chansey out, although they do not like switching into Toxic.
Even for Palkia that run Dragon Tail, Calm Mind Arceus formes are threatening enough that a check to them in some form is strongly suggested. Bulky Choice Specs and specially defensive Kyogre are fairly reliable checks to the majority of the Arceus formes that use Calm Mind. Besides direct checks, simply overloading the Arceus forme with offensive pressure from the likes of Mewtwo and Darkrai can wear it down enough to no longer threaten Palkia. Darkrai in particular is a helpful teammate as it is one of the few offensive checks to Calm Mind Arceus-Ghost, the most common and dangerous Calm Mind user, and has Dark Void to serve as a blanket solution to the other Calm Mind Arceus.
Packing a Steel-type to switch into the Dragon-type attacks that Palkia attracts is helpful to avoid losing a Pokemon every time Palkia knocks something out. Latios and offensive Latias can be especially bothersome to handle if they manage to find themselves face-to-face with Palkia. Ferrothorn, Jirachi, Arceus-Steel, and Heatran are generally reliable switch-ins to these two. Genesect is also a note-worthy check with its STAB U-turn, however, its fragility means even resisted Draco Meteors take a sizable chunk out of its health.
Spikes support is worthwhile for teams using Dragon Tail Palkia, as it allows Palkia to wear down its checks more quickly, reducing the need to predict switches as often. Forretress, Skarmory, and Ferrothorn are effective defensive Spikers that can also serve as Chansey or Arceus-Grass checks as mentioned above. Deoxys-S, Deoxys-A, and Froslass are some good examples of reliable suicide leads that can quickly set up layers of Spikes for offensive teams.