+1 0 SpA Volcarona Bug Buzz vs. 4 HP / 0 SpD Tyranitar in sand: 254-300 (74.26 - 87.71%) -- guaranteed 2HKO
+1 0 SpA Volcarona Fiery Dance vs. 200 HP / 0 SpD Landorus-T: 202-238 (54.74 - 64.49%) -- guaranteed 2HKO
Bulky Volcarona isn't a problem at all. The only problems I have with Volc are if it's offensive, carries Fire Blast, Stealth Rock isn't up, and I let it set up. Besides, what is it even setting up on other than -2 Latias? ThunderPunch does a shit ton from Conk. Jirachi is always at at least +1 when an opponent switches in, and I can win "CM wars" since I attack the physical side with Psyshock. Landorus, as I said, can tank a hit and OHKO with Stone Edge. The times I lose to Volcarona are actually few and far between. That being said, if you see a problem with the team, suggest a change and I'll test it. Lavos Spawn's suggestion for bulky TTar helps against Volc quite a bit:
+1 252 SpA Life Orb Volcarona Bug Buzz vs. 252 HP / 252+ SpD Tyranitar in sand: 283-335 (70.04 - 82.92%) -- guaranteed 2HKO
Giratina-O > Hippowdon could work too, as it can deal with the majority of weather abusers, and can semi-reliably beat Ho-oh in rain, provided it doesn't get burnt. Zekrom is revenge killed by Palkia, but if scarfed will be a pain in the ass. Hmmmmm
This seems like Ferrothorn would be a major pain in the ass, so I'd suggest you run Lustrous Orb Palkia with Spacial Rend / Surf / Fire Blast / Thunder, and I think Arceus-Ghost could give you trouble too. But it'd depend on the Kyogre set; if you're running Scald / Roar / Rest / Sleep Talk, or something similar, you should be okay. It's only a soft check, but it's pretty good. Also, I don't really see the need for Hippowdon. You beat Ho-oh with Terrakion, Palkia, Kyogre, and Stealth Rock will keep it in check.
First, you'd have to bluff a scarf,, in order to lure out Ferrothorn, which can be difficult. Furthermore, Fire Gem means Palkia loses a lot of power, as compared to Lustrous Orb, Life Orb, Choice Specs, etc. Also, Ferrothorn is almost always used in rain, so the calculations are 252 SpA Fire Gem Palkia Fire Blast vs. 252 HP / 208+ SpD Ferrothorn in rain: 292-344 (82.95 - 97.72%) -- 25% chance to OHKO after Stealth Rock, which admittedly is pretty good, but if you screw up, or they switch out, you waste your Fire Gem on most likely a resist in the rain.
I'll test it out when I have time though, but Ferrothorn is already 2HKOed by Fire Blast in rain, so it really comes down to how skilled a player you are. I suck, so :/