[TrollFreak]

Got your PM, good looking team you got here GRB1, here's my suggestions (Haven't rated teams in a while, so take these with a hint of salt)

From the looks of it, I think using the Sub-Shuffling set > your current set for Giratina will work for a couple of reasons. It helps Giratina-O evade status, as well as makes it harder to take down. Conserning the HP problems, I'll get to that in a sec. Also, use Will-O-Wisp and Shadow Sneak w/ 164 HP / 56 Atk / 252 Def / 8 SpD / 28 Spe, as you need WoW to check EKiller still, plus Shadow Sneak to check Mewtwo. The attack EVs also ensure you'll break 248/0 Ho-Oh's subs, which is great as without Outrage, it'll be hard to check Mewtwo with this set. Also, I think it might work better if you used Focus Blast > Dragon Pulse on Resh. This helps to take care of T-Tar, which if it gets sand up, could be troublesome due to Excadrill. It is inaccurate, so I understand if you don't want to use it. Finally, I think using Stone Edge > Fire Punch on Groundon helps, as you can now deal with Ho-Oh better, which your team is weak to.

Speaking of Ho-Oh, Reshriram and other powerful sun powerhouses might be troublesome for you. As such, using Arceus-Water > Steel helps as it can take on both Resh and Ho-Oh and help deal with Kyogre. This will also give your team a better defensive backbone to deal with the powerful threats in the sun, as well as some in the rain. However, chaning Arceus formes means you lose your Steel-type. So, heres what I suggest. Using Jirachi> Latias is better, as you have something else to check Mewtwo as well as Kyruem-White(you know, if you don't get roasted by Fusion Flare). You also have something with Wish to help heal Giratina-O and Resh. Plus, using U-turn helps, as you can use Wish and U-turn into Gira, Resh, or whatever works best in that situation. This builds up offensive momentum, making it easy to bring in Resh and Mewtwo. I know you're thinking losing Latias means no Ogre switch in, but since you're using Arc-Water, you'll still have a switch in.

So, in summary:

Sub-Shuffling w/ WoW and Shadow Sneak > Current Gira-O set

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Pros: Sub helps to evade status, more defensive spread to help deal with EKiller Arceus, Attack EVs ensure you break 248/0 Ho-Oh's subs

Cons: Less Attack for Shadow Sneak, no Outrage to smash Ho-Oh, though the bulk of the Sub set will work better

Focus Blast > D-Pulse for Resh

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Pros: Smash T-Tar, Arceus-Steel in rain, Arceus-Rock

Cons: Inaccurate, free switch in for Ghost-types and Wobba

Stone Edge > F-Punch on Groundon

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Pros: Gets rid of Ho-Oh, Rayquaza, the latter of which can be annoying to the team

Cons: Inaccurate like Focus Blast, but I think Stone Edge will work better for the team overall

Arceus-Water > Steel

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Pros: Ho-Oh, Resh check, Ogre switch in, can use T-Wave to spread status and clear a path for Resh, or WoW to cripple physical attackers

Cons: Losing your Steel-type, though Jirachi covers this base well

Jirachi > Latias

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Pros: Wish support for Gira-O and Resh(latter doesn't have Leftys, former hates entry hazards and you don't have a spinner), U-turn to build momentum and pass wishes, T-Wave/Body Slam to slow down faster pokemon, Kyurem-White check

Cons: Losing a ground and fight resist, but Gira-O has you covered on those bases, and Arc-Water makes a good Ogre switch in as well

Sets

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Giratina-O @ Griseous Orb
Trait: Levitate
EVs: 164 HP / 56 Atk / 252 Def / 8 SDef / 28 Spd
Impish Nature (+Def, -SAtk)
- Substitute
- Dragon Tail
- Shadow Sneak
- Will-O-Wisp


Arceus-Water @ Splash Plate
Trait: Multitype
EVs: 252 HP / 200 SDef / 56 Spd
Calm Nature (+SDef, -Atk)
- Recover
- Ice Beam
- Thunder Wave / Will-O-Wisp
- Grass Knot / Flamethrower


Jirachi @ Leftovers
Trait: Serene Grace
EVs: 252 HP / 224 SDef / 32 Spd
Careful Nature (+SDef, -SAtk)
- Wish
- U-turn
- Thunder Wave / Body Slam
- Iron Head

Good luck with the team, hope I helped :D