Well, a late answer and a half xD.
Now, I'm not the most experienced in Ubers, Ou is my thing, but I can help you fix a couple of things with your synergy and mistakes.
Metagross
I can see what Meta was used for here. It's a powerful tank that can hit hard. The moves are a bit off but I see not that much wrong with Meta, pretty standard set. Although what was Brick Break for? Perhaps replace it for Zen Headbutt for a powerful secondary STAB or Bullet Punch for some Priority.
If you want a hard hitting Steel type you could always opt for the much more powerful Arceus-Steel, which carries some useful resistances while maintaining offensive pressure, and can learn Ice beam for a true anti-dragony feel. I will give a Physical and Special set for the God of Iron.
Physical
Arceus (Arceus-Steel) @ Iron Plate
Trait: Multitype
EVs: 252 Atk / 252 HP / 4 SDef
Adamant Nature
- Recover
- ExtremeSpeed
- Stealth Rock
- Earthquake
Physical support set. You can take out SR if you wish, as I will be shortly making suggestions on your Deo-d that involves Hazards. for now, you can replace it with perhaps will.O.wisp. Iron head can be used for a full bulky attacker. If you go with W.O.W replace those Def Evs and turn them into SpDef.
Special
Arceus-Steel @ Iron Plate
Trait: Multitype
EVs: 222 SAtk / 186 SDef / 100 Def
Modest Nature
- Judgment
- Ice Beam
- Earth Power
- Calm Mind / Will.O.Wisp
This is a more unusual set among Arceus-Steel. it is one hell of a special tank though. It laughs at Garchomp's and Rayquaza's Outrage, and even more so at Reshiram's Specs Boosted Draco Meteor. for even more anti Dragon, Ice beam will defeat a plethora of Dragons bar Reshiram and Kyurem. But Steel Judgement will Kill Kyu, and Resh will bow down to it's hard hitting Earth power. Both of which Arceus will outspeed if they lack full speed EVs and natures.
Luckily Arceus has a very large movepool to pay with. So you can go with an all out bulky attacker set, an all out support set or anything inbetween. Psyshock should be mentioned for sake of the Pink blob, but not necessary for sweeping. Roar is very good for Phasing set up sweepers and abusing hazards.
Kyurem-White.
Kyu has immense special attack and HP, making it a very poten tank. It's ice moves can easily take care of Ground types bar Excadrill with ice beam. It can make a very good core with Arceus-Steel or Metagross (if used with Arceus steel Arc should use a Physical or support set) taking care of each other and tanking threats. The trouble is thouhg, that speed can really make Kyu lack. Hard hitting Dragon types will KO it wit Draco Meteor and other dragon moves. as well as many Dragons carrying fire or Ground moves for Arc. so I think you could slap a scarf on it. That will fix that speed issue (it did for Kyu-B).
Dialga
You currently have a large problem with fight types. Even with Deo and Lugia, you might not be able to cover some of them correctly. Dialga doesn't help that much with this problem. it will also leave 2 Pokemon weak to ground, and make fire somewhat problematic as well. you already have a special dragon type, you need some Physical sweepers. Rayquaza not only will stop many fight types, but is also immune to Ground and resistant to Fire.
Rayquaza @ Life Orb
Trait: Air Lock
EVs: 252 Spd / 252 Atk / 4 SDef
Adamant Nature
- V-create
- Dragon Claw
- Dragon Dance
- ExtremeSpeed
The standard Ray set. Enjoy!
Mewtwo
Despite being one of the most powerful Pokemon in existence, this mewtwo isn't being that useful. We have powerful special sweepers anyway. It Also shares some unfortunate weaknesses with Lugia, which we need around to wall many Pokemon. Right now, we have some problems with Ice. Other than Arc, it pretty much wrecks us. another thing are spinners. Rocks are important to the world of Ubers. Being able to get rid of them puts you at an advantage. There are a few usable spinners here. Ferre could help us out here. A few useful resists and a mix of Hazards will serve you nicely.
Forretress @ Custap Berry
Trait: Sturdy
EVs: 252 Def / 252 HP / 4 Atk
Relaxed Nature
IVs: 0 Spd
- Rapid Spin
- Stealth Rock
- Spikes / Toxic Spikes
- Gyro Ball / Volt Switch / Explosion
XD
Lugia
With Mew2 notwithstanding, we can have Lugia be our main wall. Your set looks OK, but nothing special. Toxic and Roost are fine. But dat Hydro pump ain't gonna get you nowhere. Ice beam is decent but isn't helping that much tbh.
Lugia @ Leftovers
Trait: Multiscale
EVs: 34 SAtk / 222 Def / 252 SDef
Bold Nature
- Roost
- Toxic
- Whirlwind
- Psyshock / Psychic / Ice beam
Da full Phaser.
Deoxys-D
That set isn't going to get you anywhere fast...or slow for that matter. Or anywhere full stop. We have a Psychic type staller right now. So we don't really need Deo-D. And other than Ray we have very little against Fire types. So what I am suggesting is Palkia, a powerful Mixed sweeper that can take Fire hits all day long.
Palkia @ Lum Berry
Trait: Pressure
EVs: 252 Spd / 252 SAtk / 4 Atk
Hasty Nature
- Outrage
- Hydro Pump
- Fire Blast
- Thunderbolt
Feel the power that Palkia brings.
Other Notes
But from what I have seen, with the exception of arc and maybe Lugia, Xtreame Killer Arceus wreaks hard. But it always does. The main and biggest counter to Arceus is Giratina. But that would add more Dark and Ghost weakness. Areus Steel with W.O.W can quickly dispatch of it, but you need back up just in case. A few other counter include Terrakion, which can survive an Extreme Speed and OHKO with a powerful Close Combat. Lugia can stop Arceus if equipped with Reflect (you can sub any move I gave for Reflect) and can then blow it away with whirlwind. if you decide to Keep Dialga then that can wall it around before giving it a powerful Aura Sphere. Though it can fight back with Earthquake.
I hope I helped at all ;D
