I'd would like to start off by saying that we (the community) have really exacerbated our Kyogre and in general weakness to rain. First off, Reshiram in itself is weak to Rain, kinda obvious, and poses little to no threat to a decently built Rain team. In Rain, Reshiram's most reliable move is Draco Meteor which is pretty much one time use. After that you're stuck with an Outrage which most physically defensive Pokemon will be able to tank easily. In Rain, Reshiram will also find it difficult to break the standard rain core. 64 HP Scarf Kyogre can also take a Draco Meteor if the need arises ~75.9% - 89.9%. Now, the more obvious response is that its a sun team so Reshiram is supposed to function in sun. However, keeping the sun up isn't really as easy as it sounds. Due to the lack of hazards on this, Kyogre can just keep switching in and out without ANY damage taken. So, its really no problem for a rain team to blot out Reshiram's precious sunlight.
This said, its all archival at the moment since we've already made our decisions. Since, the obvious winner of the poll at this moment is Paradancer Groudon, we can be assured that it poses absolutely 0 threat to a rain team especially since, Groudon is slow as hell, cant really punish Ferrothorn or Skarmory and to be very frank, its really really hard to paralyze something of note, since Pokemon like Kyogre or Palkia dont normally switch into a Groudon. So, we will be needing a very sturdy rain check and as much as I hate it, I have to say Water Arceus does seem a better choice. Like Poppy said, Water Arceus has BETTER OVERALL TYPE SYNERGY with Groudon than Palkia. More over it is far bulkier than Palkia. Granted, its main STAB gets gimped in sun but Water Arceus will be mostly functioning under rain and Flamethrower gets a pseudo STAB boost in Sun anyway.
So, I'm definitely going to agree with Poppy about using Water Arceus over Palkia due to reasons of bulk, reliable recovery, and better type synergy with Groudon. Also, let not forget, Waterceus has a lot more offensive presence in the longer run than Palkia thanks to Calm Mind. About, the mandatory Scarfer or revenge killer, its easy enough to use another one such as Zekrom or Terrakion but that actually depends on how the rest of the team pans out and what else we need to check and counter-check. So, that willl come in the later stage.