Hey there Mismagius, here I am!
As you said, Extreme Killer Arceus is very dangerous for you. It can easily set up on Forretress, and hit extremely hard as you struggle against it. A quick, solid fix would be just to change your Groudon to the Standard Support set.
EVs: 252 HP / 252 Def / 4 Spe
Impish Nature (+Def, -SAtk)
- Stealth Rock
- Stone Edge
Doing so gives you much more insurance against the physical titans of the Ubers metagame such as Rayquaza, Salamence, and the Extreme Killer. You could run Fire Punch over Roar to sneak a surprise-hit on Ferrothorn and Forretress. You could also use Rock Slide over Stone Edge if you don't want to risk a miss against Ho-Oh, but the difference in power is notable.
With Stealth Rock on Groudon, it is no longer needed on Ferrothorn. You could put either Toxic or Toxic Spikes > Stealth Rock.
Toxic allows you to poison the likes of Giratina-O, Lugia, Ho-Oh and Toxic Spikes poisons the grounded Pokemon, such as Kyogre.
Grass Arceus does not work well in sun. The sun just makes its weakness to Fire worse by boosting these moves. However by removing it you become much weaker to Kyogre, so I recommend Lustrous Orb Palkia
@ Lustrous Orb
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature (+Spd, -Atk)
- Spacial Rend
- Fire Blast
- Surf / Hydro Pump
The choice between Surf and Hydro Pump is up to you, but be careful, as both will not do much in sun. In rain however, they hit extremely hard, thanks to the additional boost from rain added to the Lustrous Orb and STAB boosts. Earth Power is also an option there to hit Heatran which can be very annoying.
Now the last thing may seem very minor, but it is not. I suggest you move 4 Speed EVs to Attack
from Giratina-O's EV spread. Doing so will ensure the 2HKO on 4/0 Mewtwo with Shadow Sneak, giving you a safer way to deal with it.
That's it, good luck!