Yeah, Palkia can't really work as a Kyogre counter on this team because it lacks hazards, so you need something that can't be slowly worn down while Kyogre switches out after every time it hits its "counter". Water Arceus seems like a good option with Recover, but I think the following should also be considered:
Latias @ Leftovers
Ability: Levitate
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature (+SpD, -Atk)
- Calm Mind
- Dragon Pulse
- Psyshock
- Recover
Latias is really only threatened by Choice Specs Ice Beam, Choice Scarf Ice Beam doesn't 2HKO even with Modest and Stealth Rock. Choice Specs full power Water Spout also 2HKOes, but if you come in on the first one, and first Psyshock / Thunder, you won't be so you can Recover the damage off. Latias has several advantages over Water Arceus:
- Its main STAB isn't weakened in sun.
Defensive Calm Mind Arceus is cool and all, but with the sun weakening its STAB move, and usually its only attacking move it has some problems offensively. In the sun, almost anything not weak to Water can come in and set up on you. For example, Blaziken, who is really frail for Uber standards and also weak to Surf, isn't even KOed and can set up a Swords Dance on Arceus and punch a hole in this team. Arceus will also leave a lot of common to set up hazards, such as Tentacruel and Forretress, which can really harm this team. I don't think this team is able to use a Rapid Spinner, as the only one that is even viable on sun teams imo—Forretress—doesn't work on this team with hazards not being allowed.
- Latias leaves room for another Arceus form.
This is useful. Water Arceus is a good Kyogre counter, but by using it this team doesn't have room for another, better Arceus form such as Normal, Steel, or Ghost anymore.
- Latias is immune to Spikes, and more importantly Toxic Spikes.
As I mentioned before, this team will very likely not use a Rapid Spinner. Now look at one of Kyogre's most common teammates, Tentacruel. If Tentacruel manages to set up two layers of Toxic Spikes, Arceus will be very crippled and unable to counter Kyogre anymore, especially considering it doesn't have access to Leftovers recovery. Refresh is an option, I know, but it will have to use Recover first most of the time, and if the opponent then switches to something that forces out Arceus you will still be statused. Also, if you do use Refresh instead, you will be forced out without being able to Recover and you will just be statused the next time you switch in. Spikes from the likes of Ferrothorn and Skarmory will also damage Arceus, but not Latias, which is also an advantage.
Latias has several options in the third slot. Psyshock is the best in my opinion, as it works regardless of weather, and it's also the best at beating other Calm Mind users, including Calm Mind Kyogre who might be a little troublesome for Latias to Calm Mind war with. Thunder is still a good option though, hitting Kyogre hard, as well as Steel-types bar Ferrothorn at least neutrally. Unfortunately it's rather ineffective in sun, so Grass Knot is also a good option for hitting Kyogre, as well as opposing Groudon.
Anyways, as I mentioned before you will need to pair this Latias with something that can take Ice Beam easily. Now, what can take an Ice Beam easlily, pairs well with Latias, and is good on sun teams?
Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 HP / 128 SpD / 128 Spe
Calm Nature (+SpD, -Atk)
- Taunt
- Toxic
- Roar
- Flamethrower
Oh right, Heatran! This thing is great, as it provides multiple great resistances, such as Dragon, Ice, and Fire. It also counters several very powerful Pokemon, including Ho-Oh, Ferrothorn, and stall teams in general. Taunt and Toxic is a great combination, as it removes any stall problems this team might have, with Taunt preventing the opposing Pokemon from recovering, while Toxic wears them down fast. Toxic is also great for hitting Kyogre switching in, neutering its Water Spout and making it much easier to deal with in general. Roar is on there because it can phaze out Pokemon that can deal with Heatran, such as a Garchomp switching or something similar. However, with this team lacking entry hazards, Hidden Power Rock is also a good option, as it makes it easier to take down SubRoost Ho-Oh. Flamethrower is in the last slot, as in sun it can do cool stuff like 3HKO Lugia. Lava Plume is still an option though, as burning Garchomp or Rayquaza switching in can be very useful.