This team has many, MANY, flaws.
Groudon: I would recommend Bulk Up over Stone Edge, to help him tank better.
Deoxys-Speed: You don't understand. He just doesn't take hits. Give him 252 Atk, 252 Spd, 4 HP, Jolly nature. Replace Recover with magic Coat/Taunt, and ExtremeSpeed for Superpower to take out Darkrai and Tyranitar. Also make him your lead.
Hitmintop: Doesn't belong in Ubers. Use a forretress.
Forretress @ Leftovers
Nature: Impish
EVs: 252 Sp. Defense (or Defense, your choice), 252 HP, 4 Spd
Trait: Sturdy
Moves: Toxic Spikes, Spikes, Rapid Spin, Volt Switch.
Classic. Sets up hazards that Deoxys-S didn't get up, Spins, then gets out of there.
Arceus: Judgement over Fire Blast for reliability. Dark Pulse over Thunder Wave, and Ice Beam over ExtremeSpeed.
Terrakion: You can't have Polishing and SDing, use Rock Polish over SD as you have the Orb. But X-Scissor to cover Psychic types and fill that spot.
Mew: Doesn't fit in this team at all. I recommend either a wall, or another speical sweeper.
Wall:
Lugia @ Leftovers
Trait: Multiscale
Evs: 252 HP, 252 Defense, 4 Sp. defense
Nature: Impish
Moves: Roost, Aeroblast, Protect, Calm Mind
Sweeper:
Shaymin-S @ Focus Sash
Trait: Pressure
EVs: 252 Sp. Atk, 252 Spd, 4 HP
Nature: Timid
Moves: Ominous Wind, Seed Flare, Air Slash, Growth
Hope I helped! :D
