Here it is, boys and girls. The most cancerous stall I have ever created for the UU tier.
Cofagrigus @ Leftovers
Ability: Mummy
Shiny: Yes
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Hex
- Will-O-Wisp
- Haze
- Pain Split
Blissey @ Leftovers
Ability: Natural Cure
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Ice Beam
- Thunderbolt
- Calm Mind
- Soft-Boiled
Quagsire @ Leftovers
Ability: Unaware
Shiny: Yes
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Scald
- Earthquake
- Toxic
- Recover
Sylveon @ Leftovers
Ability: Pixilate
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Hyper Voice
- Heal Bell
- Wish
- Protect
Mandibuzz @ Leftovers
Ability: Overcoat
Shiny: Yes
EVs: 240 HP / 216 Def / 52 Spe
Impish Nature
- Knock Off
- Taunt
- Defog
- Roost
Aggron-Mega @ Aggronite
Ability: Sturdy
EVs: 248 HP / 8 Def / 252 SpD
Careful Nature
- Heavy Slam
- Dragon Tail
- Stealth Rock
- Rest
Cofagrigus: I used cofagrigus because he can do three important things. 1) He's very physically defensive and walls a good chunk of physical attackers in UU. 2) He can burn physical attackers with Wisp to not only cripple their attack but also hinder their longevity. 3) He has access to haze, allowing him to stop set up sweepers that try to get out of control.Blissey: Blissey is your main answer to the special attackers in the tier like Hydreigon and Nidoking. A calm mind set is used as it not only can end the game instantly if your opponent lets you set up, but also because it doesn't need to play the role of the rocker or the cleric since Aggron and Sylveon do those, respectively.
Quagsire: With unaware, this gives you an immediate answer to physical setup sweeper like SD lucario. Also, it can apply pressure on your opponents with widdle damage from toxic and scald burns. And finally, the ground typing gives you an immunity to electric, allowing you to be a nice check to mons like defensive Mega Ampharos.
Sylveon: Sylveon fills the role as the designated cleric of the team. Not only that, but it can also pass wishes to mons like Mega Aggron for safer switch-ins. And of course, with the blessing of the fairy typing, it resists common dark and fighting type attacks, as well as sporting an immunity to draagon, allowing it to wall mons like Hydreigon w/out Flash Canon.
Mandibuzz: Being a ghost and dark resist, as well as having a ground immunity, allows Mandibuzz to check those heavy hitters like banded Krookodile. But that's not all it can do, it also has access to defog to be the hazard control for the team, as well as taunt to possibly prevent your opponent from setting up their hazards. And knock off is there as your only attack, which also is great for crippling your opponents Pokemon by removing their items.
Mega Aggron: Being steel type is great, allowing it to resist fairy type moves coming from mons like Sylveon and Whimsicott, and just smash 'em in return with heavy slam. He also serves as the teams rocker, and thanks to dragon tail he can phase the opponent around to rack up that residual from hazards. Finally, rest is his only form of recovery outside of receiving wishes from Sylveon, but luckily Sylveon can also heal bell to quickly get rid of that sleep condition that rest applies to Aggron.
Battle replay in my sig. Do note, however, that the game lasts well over 100 turns.