Palkia is a Pokemon that, akin to mons like Kyurem-White, Ho-Oh, or even Specsogre back in XY, simply needs to get into battle to wreak havoc. Due to this, Palkia thrives on offensive teams, and likes teams that can maintain offensive pressure to wear down its checks. Speaking of which, these checks are far and few in number - the only mons that can switch into Palkia are Dialga, Water/Grass/Fairyceus, and the blobs. With that knowledge, Mega Gengar is a no-brainer, decimating 5/6 of the above and trading with Dialga if need be. I'm running a Perish Song + Destiny Bond set on it, so that it beats those damn annoying status games with Arceus-*s, and it has the most potential to take down at least 1 mon with it, and 2 is more than just a possibility. From here, I don't hesitate to add in the obligatory Primal Groudon - the addition of Palkia allows the strain of checking Kyogre to be reduced, and gives me the freedom to run a more offensively inclined set featuring Overheat that gets rid of potential checks, particularly Yveltal, which can beat both Gengar and Palkia. At this point, opposing Double Dance Don shits on me if given the setup opportunity on Palkia or Gengar. With this knowledge, Calm Mind Latias is added to the team - it has synergy with Palkia in overloading its checks, provides a complete and absolute full stop to Primal Kyogre (like, the thing is utterly cockblocked by this team), and of course, checks Primal Don. It also serves as a nice little win condition when Gengar goes to work on its checks. Now, my biggest concerns are Dark and Ghost Arceus - they both kill AT MINUMUM one mon. I also have a noticeable weakness to opposing Mewtwo-*, Lati@s, Yveltal, Ghostceus, Ho-Oh and Darkrai. My dillemma - a Ho-Oh here would solve (or come close to solving) most of those problems. However, I wanted to maintain the offensive pressure that does not allow for any opportunity for Rayquaza or Salamence to set up. As such, I'm forced to add a mon that I don't particularly like - Fairyceus. In turn for accentuating the Darkrai and Ho-Oh weakness, it solves the Lati@s, Mewtwo and Yveltal problem for me while still killing Rayquaza and Mence. Lastly, I need a check to Ho-Oh, Darkrai and Ghostceus. The first thing I think of is Tyranitar. And then I realize - Tyranitar not only solves just about every problem on that list, but Excadrill offers another Xern check, and all of a sudden, literally FUCK EVERYTHING I'VE DONE UP TO THIS POINT. I tear everything you just wasted your time reading down, and start over with a squad of Tyranitar / Excadrill / Palkia / Mega Gengar. Now, Tyranitar is a blanket check to literally everything, obligatory Excadrill, and Palkia + Mega Gengar was already explained. From here, I throw back on that Latias (non CM) for reasons explained above. No CM this time, so I can focus on it helping defend against the Primals and also provide Healing Wish and Defog support for Tyranitar/Palkia. It also has good offensive synergy with Palkia, and beats a bunch of the miscellanous Arceus formes that would otherwise give me trouble. At this point, if we're doing purely theorymon, the only real threat to this team is Ghostceus, be it Swords Dance or Calm Mind. However, with the team structured the way it is, Tyranitar is impossibly overloaded with things it needs to check, so I need an Arceus-Forme that checks Ghostceus whilst relieving Tyranitar of some pressure. The only mon fitting the bill is Darkceus. Darkceus takes the pressure of Yveltal, Darkrai, Lati@s and, to some extent, Mewtwo-Y off of Tyranitar, allowing it to focus on the other threats.
What I take away from this experience - holy fuck it's hard to build with Palkia. Like, seriously, you HAVE to try to build a team that accentuates its niches, because if not, it's merely outclassed dead weight. This was painful af. ._.