Do you know what order those pokemon are in?
if not
Use this Team =]
Metagross @ Life Orb
EV Spread 112 HP/252 ATK/16 DEF/ 132 Spe
Adamant
Meteor Smash
Ice Punch
Agility
Ice Punch
This Set is good If you know the Mewtwo is going up first.
You Use Agility first, because most Mewtwo Sets do not have FlameThrower.
Actually, I have never seen a Mewtwo with Flamethrower/fire blast.. (not a good one anyways)
So you have your +2 speed, You go on and KO or near KO his Mewtwo with a Life Orb STAB Meteor Mash.
If he retreats to Garchomp, you will Outspeed it with your Agility and use Ice Punch,
If the Ttar, then you just Meteor Mash.
Since you have 112 HP Evs you will most likely be on low HP after a few hits, Just Explode, as you know you are ahead..
Even by just knowing his team.
Hippowdon @ Leftovers
EV Spread 252 HP/ 168 DEF / 88 Sp Def
Impish
Ice Fang
Slack Off
Earthquake
Curse
Once the Mewtwo is down, You can Wall the rest of their pokemon.
Use Curse and Slack off to raise its Defences and Attack, while they try to 2HKO you
You can run through the Opponent's Ttar and Garchomp.
Just be aware that your attacks CAN miss Garchomp because of its Sand Veil.
Make sure you don't risk it entirely.
Gyarados @ Life Orb/Choice Band (depending on Item Clause or not)
EV Spread 252 ATK / 4 DEF / 252 Speed
Adamant
Waterfall
Dragon Dance/Taunt
Earthquake
Ice Fang
Intimidate can make their Physical attacker very weakened, and it can help you set your Hippowdon up (curse)
Good luck =]
i'm gonna have dinner now =p