As requested, here's my rate Alex :) Ofc, you had alot of sucess as is, so you don't need to take any of these suggestions.
On Groundon, if you're going to use Max Attack and Speed, just use
Adamant, as you don't need to max on Speed when using Jolly. However, if you do want to run Jolly, a spread of
96 HP / 252 Atk / 160 Spd works, as you outpace neutral natured base 90s without losing significant bulk. For Giratina-O, while Poppy's suggestion of Outrage works wonders, imo,
Will-O-Wisp > Shadow Sneak with a spread of
164 HP / 188 Atk / 72 Def / 84 Spd will work better, as you can cripple Pokemon such as SD Arceus, which can be a threat since you don't have a physical tank / D-Bond on Frosslass, and with that spread, you can survive a SD Arceus's Shadow Claw and either burn it or phaze it out. Also, with that spread, you can make 4 Subs, and you have enough Speed to outpace Tentacruel, Dialga, and Mold Breaker Exca so you can hit them with EQ. Finally, using a
Choice Scarf on Palkia with
Thunder > Dragon Tail and a spread of
252 SAtk / 4 SDef / 252 Spd works much better, as it gives your team a revenge killer than can revenge kill things that Zekrom can't(faster dragons, for example). This also gives you a way to deal with Manaphy in the rain, which can be troublesome. The spread means you take less damage from entry hazards, so thats always nice.
From there, I start changing members. Jibaku brought up some great points, and there is a Pokemon that can deal with most of those threats,
Stallbreaker Heatran > Zekrom works wonders here, since you would have a Choice Scarfed Palkia + Zekrom is so slow for a scarfer in the current meta, plus Heatran gives you a Dragon resist that can deal with Dialga w/o Earth Power + Aura Sphere(which is rare on Dialga), makes a great Kyurem-White/Palkia switch in(Palkia in Sun, as Surf won't 2HKO), Ho-Oh without EQ, Giratina-O without EQ, Arceus-Ghost w/o Focus Blast or Brick Break, Arceus-Dark, as well as loving Sun from Groundon. It can also help break down Skarmory + Ferrothorn, taking pressure of of Groundon to handle those Pokemon. Plus, with Roar, you can help spread around entry hazard damage, which is helpful since this is SpikeStacking offense. From there, you could use something with Speed to keep up the pressure, so
Mewtwo > Arceus works, as it deals with Arceus-Fighting and Mewtwo packs the offensive pressure that both Poppy and Jibaku suggested, as well as better coverage than Arceus. Now of course, losing Arceus is hard, especially with SpikeStacking offense, but since 2 people have suggested it that have much more experience than me, try it out.
So, in summary:
Heatran > Zekrom
Pros: Dragon-resist, Fire-immune, Dark-resist(huge for Darkrai) handles Pokemon such as Dialga, Ho-Oh, Arceus-Dark and others easily unless they run Earth Power or EQ, so its always best to scout for those first before trusting Heatran, Toxic to break down Pokemon such as Lugia and Kyorge switch ins, Roar to spread damage
Cons: Losing a reliable check to Mewtwo + losing a Darkrai check, though Heatran can beat Mewtwo w/o Aura Sphere + Dakrai w/o Focus Blast, and if they do have those moves, thanks to SpD investment, Heatran can take an attack(Aura Sphere nor Focus Blast OHKO) and phaze them out
Sets
Heatran (M) @ Leftovers
Trait: Flash Fire
EVs: 248 HP / 152 SDef / 108 Spd
Calm Nature (+SDef, -Atk)
- Taunt
- Toxic
- Roar
- Flamethrower
Mewtwo @ Life Orb
Trait: Pressure
EVs: 4 HP / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
- Psystrike
- Fire Blast
- Aura Sphere / Shadow Ball
- Substitute / Shadow Ball / Calm Mind
Good Job on getting #1, hope my rate helps!