Well, I ended up changing a fair amount of your team. The only pokes that stayed almost completely the same are Blissey and Skarmory. Hope you don;t mind, but in the end it came out to a very, very solid team.
I took your original team and started battling with it on main. Giratina-O was switched as the team needed a Scarfer. I put in Genesect who hits most of the tier and is one of the best Scarfers in the Metagame. I then enlisted the help of a friend, who pointed out that Dialga could carry rocks (as he does this better then being an all-out attacker) which meant I could switch Deo-s for someone with more utility. Latias came in as a hardcore check to Kyogre. However I then came to a Stall Breaker Sun team and was utterly destroyed. And so in came Mewtwo's replacement, Arceus-Ghost equipped with a move to hard-counter Ho-oh specifically. I also changed Dialga's role and set completely. So without further rambling, my rate for your team.
We'll start with the remaining members of your team :P
Skarmory @ Leftovers
Trait: Sturdy
EVs: 252 HP / 252 Def / 4 SDef
Impish Nature
- Roost
- Brave Bird
- Whirlwind
- Spikes
So same everything except for the EVs. I changed them to fully Defensive, this mean Blissey can fully abuse it's Bulk and SDef and have someone to take pretty much any physical attack in the metagame.
Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 4 Def / 252 SDef
Careful Nature
- Wish
- Toxic
- Heal bell
- Seismic Toss
I changed the EVs to full SDef and switched out Snatch for Seismic Toss. There was one time in the 20 or so battles where I could have used Snatch, and the remainder I used Toss. So definitely a change that should be made.
Dialga @ Lum Berry
Trait: Pressure
EVs: 252 HP / 56 SAtk / 200 SDef
Modest Nature
- Dragon Pulse
- Fire Blast
- Stealth Rock
- Roar
Anti-lead Lum Dialga. More necessary bulk and with enough SAtk to hit hard. Dragon Pulse preferred over the normally seen D-Meteor so you can stay in and still hit hard. Fire Blast hits Genesect, Ferro, and Forretress all hard, even in the rain. Roar gives you an additional phazer which is nice to have in a tier where so many pokemon set up and sweep. Stealth Rock is ofc hazards, and works really well against leads that start with status. Darkrai in particular. It also functions well as a status absorber later in the game. You could also go for Leftovers if you don't like the Lum.
Genesect @ Choice Scarf
Trait: Download
EVs: 252 Atk / 4 SAtk / 252 Spd
Hasty Nature
- U-turn
- Explosion
- Ice Beam
- Blaze Kick
A Genesect that I particularly enjoy using. Blaze Kick is the crux of the set, replacing Iron Head (which does pretty much nothing other then hitting Terrakion) Blaze Kick OHKOs other Genesect in rain, 2HKOs Forretress in Rain (and without rain if Sturdy isn't broken), Genesect gets 3HKO in rain (dumb Ferrothorn) but is 2HKOd outside of rain. U-Turn is your ultimately main move, gaining momentum and making prediction far easier for your team. Ice Beam is pretty much required coverage. Explosion can deal incredible damage and with Genesect outspeeding most pokemon it'll hit hard when you're right at the end of your health.
Latias @ Soul Dew
Trait: Levitate
EVs: 112 HP / 204 SAtk / 192 Spd
Timid Nature
- Roar
- Roost
- Draco Meteor
- Thunder
Latias is your hard-counter for Kyogre which every team needs. Blissey can be a check, but it can't take max HP, Specs Water Spout. Latias however, can not only take it, but can 2HKO with Thunder, the main point in carrying it (Thunderbolt isn't quite as good if you prefer always being able to use the move instead of relying on other teams carrying Kyogre [which most do in Uber]). Draco Meteor dents everything except Steel types. With full hazards down you can cause much rage with Roar, and Roost is your recovery. Watch out for opposing Genesects. They love a good tasty Latias to practice U-Turning on.
Arceus-Ghost @ Spooky Plate
Trait: Multitype
EVs: 252 HP / 4 Atk / 252 Spd
Timid Nature
- Judgment
- Will-O-Wisp
- Recover
- Stone Edge
Arceus-Ghost is your spin blocker, and support poke. Stone Edge is there to take out Ho-oh which threatens most of the team. WOW, Recover and Judgement make up for one heck of a staller. The Ghost typing also helps the synergy on your team, as fighting was starting to become an issue, and Giratina-A brought an additional dragon weakness.
So that's my edit for your team. You had a good team to start with, but the moment you get into the higher/better players in Ubers it started to fail. And that's something I notice quite frequently in Ubers. There are teams that function well in the lower ladder that can't function at all in the higher part of the ladder. Good teams are rarely seen in the lower part of the ladder so but they generally do well there. So now you have a strong team, more stall then offense, which can be a problem if your battle style is more offense biased. In any case, I wish you luck in your future endeavors.
Importable Copy:
Latias @ Soul Dew
Trait: Levitate
EVs: 112 HP / 204 SAtk / 192 Spd
Timid Nature
- Roar
- Roost
- Draco Meteor
- Thunder
Skarmory @ Leftovers
Trait: Sturdy
EVs: 252 HP / 252 Def / 4 SDef
Impish Nature
- Roost
- Brave Bird
- Whirlwind
- Spikes
Genesect @ Choice Scarf
Trait: Download
EVs: 252 Atk / 4 SAtk / 252 Spd
Hasty Nature
- U-turn
- Explosion
- Ice Beam
- Blaze Kick
Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 4 Def / 252 SDef
Careful Nature
- Wish
- Toxic
- Heal bell
- Seismic Toss
Arceus-Ghost @ Spooky Plate
Trait: Multitype
EVs: 252 HP / 4 Atk / 252 Spd
Timid Nature
- Judgment
- Will-O-Wisp
- Recover
- Stone Edge
Dialga @ Lum Berry
Trait: Pressure
EVs: 252 HP / 56 SAtk / 200 SDef
Modest Nature
- Dragon Pulse
- Fire Blast
- Stealth Rock
- Roar
