Yo Donkey, congratulations on building such an amazing Uber team man! This team is without a doubt one of the best Uber teams I have ever seen for the ladder. I just read through the comments and realized that one of the most annoying Pokemon in the metagame hasn't been mentioned yet: Stalltwo. The stall varient of what is perhaps the most frightening special sweeper in the game can do a ton of damage to this team. Being such a fast abuser of Taunt, Stalltwo immediately forces Giratina (your primary Mewtwo switch-in) to Struggle, meaning Giratina is no longer an option. Tyranitar and Ferrothorn don't stand a chance against Mewtwo's Taunt + Will-O-Wisp + Recover combination. Gliscor needs his Toxic Orb activated to be able to defeat Stalltwo, which isn't done so easily since it can't risk switching in because Stalltwo could Will-O-Wisp on the switch and your Gliscor doesn't have Protect either. Excadrill can't switch in, and an Earthquake only does a mere 48 to 57% to Mewtwo. This is assuming that Mewtwo runs a specially defensive set. Mewtwo can then Will-O-Wisp to Recover off the damage the next turn. This means it all comes down to Arceus-Rock, who won't appreciate dealing with Stalltwo either. Ofcourse, this set is uncommon, but it's definitely a big threat and a Mewtwo set that should be used more.
Outside of the threats that have been mentioned plenty of times (*cough*Arceus-Fighting*cough*) there is one more I want to mention. Choice Scarf bluffing Palkia. Whether it's Lustrous Orb, Haban Berry or Expert Belt, as long as it carries Hydro Pump, Fire Blast and Spacial Rend it is a huge threat. Not only does it do a huge number to your team, you don't have any Dragon-type attacks of your own, meaning you can't OHKO opposing Palkia, which could be problematic. Especially since it destroys your entry hazard using Pokemon with the appropriate move. Palkia is easier to handle than Stalltwo though, seeing as hazards and Sandstorm will quickly bring it into Excadrill's Earthquake's killing range.
Outside of the aforementioned threats such as certain Calm Mind Arceus formes and Substitute + Swords Dance Garchomp I think this team is incredibly solid and especially well designed against common threats. I would recommend trying to squeeze in a Ho-Oh somewhere, but I'm not sure if you should change anything at all. Ho-Oh is amazing against most Calm Mind Arceus and other common threats to Sand. It also helps you abuse opposing Groudon's weather and relieves the Pressure on Giratina as a special wall.
Again, congratulations on building this fantastic team! (:
Choice Scarf bluffing Palkia is a big threat early on and if sand isn't up. Usually though I can wear it down with good prediction, and I've started using CM Arceus-Rock to help neutralize the threat. In rain, Ferrothorn can check it fairly well and bring its health down.
Stalltwo (and mewtwos in general) are pretty big threats. It's hard for me to wear stalltwo down. Life orb exca eases the problem slightly, but it can still come in later and do more damage.
Thanks for the rate, and I appreciate the support!
I really don't see how stuff like DD quaza, specs ogre (yes ferrothorn is like a really risky check) and non choice palkia is really dealt with, personally I think u need to replace gliscor with something like lugia as u have stacked like 5 SS resist pokes. Lugia also removes much of the fight arceus weakness and can stall easily as well as gliscor.