Ferrothorn
You might already have done this by the time this answer comes up because I mentioned this on chat, but Ferrothorn should use a spread of 252 HP / 48 Atk / 208 SpD which is the standard spread for Uber Ferrothorn.
Also, to be perfectly honest, your team doesn't really take advantage of the paraysis Ferrothorn and Groudon spreads.
Deoxys is already super fast, Palkia is scarfed, Ho-Oh is max speed, and so is Arceus. Do you really need the move?
Protect or Gyro Ball should be solid replacements, Protect is able to scout and racks up Leech Seed damage while Gyro Ball gives coverage on Dragons. If you use Gyro Ball, make sure to give Ferrothorn 0 Spe IVs and a Sassy nature.
Groudon
With Ferrothorn's specially defensive spread, Ho-Oh's natural excellent bulk on the special side, and Palkia with a pretty decent SpD and some HP investment, I suggest using the standard, support, Groudon.
Groudon @ Leftovers
EVs: 252 HP / 252 Def / 4 Spe
Nature: Impish
- Earthquake
- Stone Edge/Fire Blast/Fire Punch
- Stealth Rock
- Roar/Dragon Tail/Thunder Wave
Earthquake is STAB, and does respectabe damage when not resisted.
Stone Edge hits Ho-Oh hard, and gives Groudon the infamous, EdgeQuake combination.
Fire Blast hits Ferrothorn, Forretress, and Skarmory hard if those are important targets, and also avoids Iron Barbs damage from Ferrothorn. If you really want, you can go with Fire Punch.
Stealth Rock is hazards, and doesn't really need much explaining does it?
Roar is nice phazing, and allows Groudon to phaze out dangerous threats such as Rayquaza, and also rack up SR damage, making it easier for your teammates to clean up.
Dragon Tail does damage, but I personally don't like to risk the 90% accuracy, and Roar also has more PP, which is useful in Ubers because of Game Freak spamming Pressure on most legendaries.
If you still want to Thunder Wave things, I recommend you use it on one Pokemon as opposed to two.
This Groudon also gives you more to handle physical titans in the Uber metagame, like EKiller Arceus.
Arceus
I think Arceus is pretty likely to be revenged killed if you're using Outrage, while I do see the point of it.
More bulk can be added with a spread of 88 HP / 252 Atk / 168 Spe Adamant which has just enough speed to outpace Rayquaza, but you probably already know this.
Dragon Claw is weaker, but it might come to use since it has the same coverage, but doesn't lock Arceus in which again I don't think is very smart.
Deoxys-A
Thunderbolt is pretty much too weak for Ubers, I suggest you at least try out Thunder.
A x2 SE Ice Beam is weaker than a STAB Phsycho Boost, you should consider replacing it with Shadow Ball or ExtremeSpeed, Shadow Ball hits Mewtwo which might actually be very troublesome, OHKOing after a bit of prior damage. while ExtremeSpeed is excellent priority. You could also use both forgoing Thunder.
You target more specific targets in Uber since it isn't a very big tier, so Idk about BoltBeam.
Ho-Oh
I personally use a spread of 208 HP / 252 Atk / 48 Spe on Ho-Oh which works great, I suggest you try it out.
If you still want to use max speed, at least get rid of those 4 HP EVs to be able to swich into the field with SR up 2 times without fainting.
Also, a spinner should be used when using Ho-Oh so this is the guy for the job:
Forretress @ Leftovers
Ability: Sturdy
EVs: 252 HP / 4 Def / 252 SpD
Nature: Careful
- Toxic Spikes
- Rapid Spin
- Volt Switch/Spikes
- Gyro Ball/Spikes
You might want to replace this guy with Ferrothorn and rely on Palkia for Kyogre, but this set is here just to try out in place of whichever teammate you find least useful.
If you use this guy instead of Ferrothorn, go with spikes instead of Volt Switch or Gyro Ball.
Palkia
Just go with Thunder instead of Thunderbolt to hit Kyogre harder, plus the sun won't be up all the time anyway.
Also, a water type move instead of Ice Beam can be nice, to have more against rain teams, especially if you replace Ferrothorn not to mention Ice Beam is redundant with Dragon STAB.
Palkia has key base 100 speed, so max out its speed and give it a timid nature and put the 4 leftover EVs into HP.
Just a few suggestions, hopefully they'll come to some nice use.
Good luck with the team!
