Let me see....
Your team has quite a few weaknesses which I shall try to patch up in this rate I'm about to give now. First things first, as ItsGuillotine has stated, Mew is not very viable in Ubers as its stats are considered low by Ubers Standards, and it is setup bait for many dangerous Uber threats. Moreover, that set is completely shut down by Taunt.
Almost your entire team is made up of Special Attackers, and they are walled by any dedicated Special wall out there. The first thing I will suggest is to replace Mew with Specially Defensive Primal Groudon, as with your current team, its completely destroyed by:
Power Herb Geomancy Xerneas just sweeps your team once it nabs a Geomancy, as long as Stealth Rocks are on the field:
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 252+ SpD Mew: 331-390 (81.9 - 96.5%) -- 62.5% chance to OHKO after Stealth Rock
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 0 SpD Yveltal: 932-1100 (237.1 - 279.8%) -- guaranteed OHKO
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 4 SpD Mega Mewtwo Y: 390-460 (110.4 - 130.3%) -- guaranteed OHKO
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 4 HP / 0 SpD Arceus-Electric: 391-462 (102.3 - 120.9%) -- guaranteed OHKO
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 4 HP / 0 SpD Fairy Aura Xerneas: 466-550 (118.2 - 139.5%) -- guaranteed OHKO
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 252+ SpD Giratina-O: 662-780 (150.1 - 176.8%) -- guaranteed OHKO
Specially Defensive Primal Groudon can check Geomancy Xerneas once it gets to +2:
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 252+ SpD Primal Groudon: 177-208 (43.8 - 51.4%) -- 9% chance to 2HKO
It only has a 9% chance to 2HKO while Primal Groudon easily 2HKOes even with 4 Atk investment:
4 Atk Primal Groudon Precipice Blades vs. 104 HP / 32 Def Xerneas: 220-259 (52.5 - 61.8%) -- guaranteed 2HKO
Alternatively you can run Thunder Wave to paralyze it and cut its sweep short for sure.
Heres a set:
Groudon @ Red Orb
Ability: Drought ===> Desolate Land
EVs: 252 HP/ 4 Def/252 SpDef
Careful/Sassy Nature
-Precipice Blades
-Rock Slide/Lava Plume
-Stealth Rock
-Roar/Thunder Wave/Toxic
This Primal Groudon set also allows it to counter Primal Kyogre (as long as Desolate Land is active), as it can only do this in return:
252+ SpA Primal Kyogre Hidden Power Ground vs. 252 HP / 252+ SpD Primal Groudon: 144-170 (35.6 - 42%) -- guaranteed 3HKO
252+ SpA Primal Kyogre Ice Beam vs. 252 HP / 252+ SpD Primal Groudon: 107-126 (26.4 - 31.1%) -- guaranteed 4HKO
4 Atk Primal Groudon Precipice Blades vs. 4 HP / 0 Def Primal Kyogre: 237-280 (69.2 - 81.8%) -- guaranteed 2HKO
Every team needs a switch to Kyogre, and Primal Groudon is a perfect switch into it.
Yveltal can act as a solid Physical Wall, an idea of how bulky Physically Defensive Yveltal is that it is bulkier than fully invested Skarmory.
252 Atk Mega Gallade Close Combat vs. 248 HP / 252+ Def Yveltal: 175-207 (38.4 - 45.4%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Mega Gallade Close Combat vs. 252 HP / 252+ Def Skarmory: 133-157 (39.8 - 47%) -- guaranteed 3HKO after Leftovers recovery
This allows it to check the omnipresent Extreme Killer Arceus as it fails to OHKO even at +4,
+4 252 Atk Life Orb Arceus Extreme Speed vs. 248 HP / 252+ Def Yveltal: 359-422 (78.9 - 92.7%) -- guaranteed 2HKO after Leftovers recovery
And all you have to do is to click Foul Play. BAM, dead Arceus. Instead of Leftovers you can run Rocky Helmet to guarantee the OHKO with Foul Play after Arceus hits it with +2 Extreme Speed. Heres a set:
Yveltal @ Leftovers/Rocky Helmet
Ability: Dark Aura
EVs: 248 HP/252 Def/8 SpDef
Bold/Impish Nature
-Foul Play
-Taunt
-Toxic/Sucker Punch
-Roost
Use Impish if you are running Sucker Punch. Dark Aura Sucker Punch easily OHKOes Mega Mewtwo Y even without any Investment in Attack.
I will give Arceus the Silk Scarf and run the ExtremeKiller set as a pseudo revenge killer and a win-con. That set is notorious and common in Ubers for a reason.
Arceus @ Silk Scarf
Ability: Multitype
EVs: 252 Atk/4 SpDef/252 Spe
Adamant Nature
-Extreme Speed
-Swords Dance
-Earthquake
-Shadow Claw
Xerneas should be running Power Herb if you are going to run a Geomancy set, as Power Herb helps to allow Xerneas to skip the charging turn in Geomancy and instantly obtain its boosts. It will act as the second win-condition in your team. Heres a set,
Xerneas @ Power Herb
Ability: Fairy Aura
EVs: 104 HP/32 Def/252 SpAtk/120 Spe
Modest Nature
-Moonblast
-Thunder/Thunderbolt
-Focus Blast
-Geomancy
I would give Defog to Giratina-Origin in order to provide a way to get rid of Dual Screens and Entry Hazards:
Giratina-Origin @ Griseous Orb
Ability: Levitate
EVs: 180 Atk/252 SpAtk/76 Spe
Rash Nature
-Draco Meteor
-Shadow Sneak/Shadow Force
-Dragon Tail
-Defog
This is a mixed Attacker Giratina set, it can phaze with Dragon Tail while hitting hard with Shadow Force and Draco Meteor, while providing Defog support to the team.
Hope I helped! With the changes, the team will be more balanced in general.