Well, here it is, my first RMT. I lack Shoddy so I haven't actually tested it for myself yet, but I've thought it over for long periods of time and have good reason to believe it should perform well. Ironically, this team is heavily offense-based, even though I tend to prefer defense. The fact is mainly because I love Mewtwo and wanted to use it as it is used best: a special-sweeping monster. This led to using Groudon to boost its Fire Blast and then to using others who benefited from the weather. Eventually, I had this.

Groudon @ Leftovers
Nature: Impish
Ability: Drought
252 HP/252 Def/6 Sp. Def
-Stealth rock
-Earthquake
-Dragon Tail/Roar/Stone Edge
-Toxic/Thunder wave
Nature: Impish
Ability: Drought
252 HP/252 Def/6 Sp. Def
-Stealth rock
-Earthquake
-Dragon Tail/Roar/Stone Edge
-Toxic/Thunder wave
Groudon’s massive physical stats allow it to wall threats like Rayquaza and Zekrom whilst dealing out damage of its own. I'm using Groudon as a lead due to its effectiveness as one and to set up eternal sun early in the match. Earthquake allows it to deal damage even when Taunted. Dragon Tail cannot be Taunted but it also can't break through Substitutes, a reason for using Roar instead. Stone Edge allows it to do damage on the Flying types whom are immune to Earthquake. Toxic breaks down non-Steel walls and Thunder Wave cripples opposing sweepers.

Reshiram @ Life Orb
Nature: Hasty
Ability: Turbo Blaze
4Atk/252SpAtk/252Spe
-Nitro Charge
-Blue Fire
-Draco Meteor / Dragon Pulse
-Outrage / Dragon Claw
Nature: Hasty
Ability: Turbo Blaze
4Atk/252SpAtk/252Spe
-Nitro Charge
-Blue Fire
-Draco Meteor / Dragon Pulse
-Outrage / Dragon Claw
Reshiram is an absolute monster in sunlight. Nitro Charge comes off of its overshadowed yet still very impressive base 120 Attack stat, boosting its speed to outmatch most unscarfed opponents. Blue Fire deals terrifying damage in the sun, even 2HKOing Blissey with Stealth Rock and Spikes (Which it lacks on this team) support. Draco Meteor and Outrage deal great damage, though their drawbacks may make it wiser to utilize the weaker Dragon Pulse and/or Dragon Claw.

Mewtwo @ Life Orb
Nature: Timid
Ability: Tension/Pressure
4HP/252SpAtk/252Spe
-Calm Mind
-Psycho Break
-Fire Blast
-Aura Sphere
Nature: Timid
Ability: Tension/Pressure
4HP/252SpAtk/252Spe
-Calm Mind
-Psycho Break
-Fire Blast
-Aura Sphere
Many have called Psycho Break overhyped. A Psycho Break coming off of a CM'd Mewtwo's God-like Base 154 Special Attack with STAB deals insane damage. Even when unboosted it can 2HKO the three Pokémon that resist the attacks above. Fire Blast murders most Steel types and damages Psychics and Groudon. Aura Sphere is a great move all around and deals damage to Dialga, Tyranitar, Arceus and Darkrai. Tension is the preferred ability, as Pressure isn't as useful on an offensive set like this, but that won't be available until it is released in the Dream World.

Giratina-O @ Griseous Orb
Nature: Naughty
Ability: Levitate
252Atk/252SpAtk/4Spe
-Draco Meteor
-Shadow Sneak
-Hidden Power Fire
-Dragon Claw
Ability: Levitate
252Atk/252SpAtk/4Spe
-Draco Meteor
-Shadow Sneak
-Hidden Power Fire
-Dragon Claw
Giratina-O works just like last generation. Its STAB combination hits the majority of Ubers for super-effective damage while Hidden Power Fire destroys Steel types such as Scizor and Forretress. Shadow Sneak has the added benefit of picking off weakened opponents (mainly Psychics) and guarantees a 2HKO on offensive 4/0 Mewtwo. Draco Meteor and Dragon claw deal damage on any non-Steels on both sides of the offensive spectrum. Levitate and its Ghost typing make it remain the greatest spin blocker in all of Ubers

Jirachi @ Choice Scarf
Nature: Jolly
Ability: Serene Grace
4HP/252Atk/252Spe
-Ice Punch
-Iron Head
-U-Turn
-Trick
Nature: Jolly
Ability: Serene Grace
4HP/252Atk/252Spe
-Ice Punch
-Iron Head
-U-Turn
-Trick
Jirachi works incredibly well as a revenge killer in the Uber tier. Ice Punch hits almost every Uber Dragon for Super Effective damage, and has a 20% Freeze chance with Serene Grace. Iron Head is a wonderful STAB move with a great 60% flinch rate on anything Jirachi's faster than, which, with the Choice Scarf, is almost every nonscarfed Uber. U-Turn hits the many Psychics in Ubers for Super Effective damage as well as Darkrai and Tyranitar. Trick allows Jirachi to cripple Walls and steal their item.

Darkrai @ Life Orb
Nature: Timid
Ability: Bad Dreams
4HP/252SpAtk/252Spe
-Dark Void
-Nasty Plot
-Dark Pulse
-Focus Blast
Nature: Timid
Ability: Bad Dreams
4HP/252SpAtk/252Spe
-Dark Void
-Nasty Plot
-Dark Pulse
-Focus Blast
Darkrai is an incredible special sweeper whose main flaw is its mediocre defenses. Dark Void makes Darkrai almost uncounterable. Its 80% accuracy is phenomenal for a Sleep-inducing move, only beaten out by the 100% accurate Spore. Dark Void breaks Sashes due too Darkrai's ability, which takes off 12.5% of a sleeping opponent's HP each turn. It also allows the Nightmare King to easily set up Nasty Plot, whether the opponent tries to wake up or switches (Which would reset their sleep counter). Dark Pulse is a great STAB move in the Psychic-infested Ubers and does massive damage with the Nasty Plot and Life Orb boosts. Focus Blast hits almost everything that Dark Pulse doesn't and allows Darkrai to 2HKO Blissey after a Nasty Plot. The only problem is the shaky 70% accuracy.
Well, there it is. The main problems I'm seeing with it are the fact that my Ghost/Dark resist is the fragile Darkrai, lack of a Rapid Spinner, lack of a Sleep Talker to absorb the Dark Void of an opposing Darkrai, and the sunlight makes Jirachi even more susceptible to Fire attacks. Kyogre is probably the biggest threat to my team because it removes the sunlight which Groudon sets up and the closest thing I have to a counter for it is Mewtwo, who's Psycho Break can OHKO an offensive Kyogre after a Calm Mind.