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Normally, I'd make Tyranitar a setter-upper of Stealth Rock, but since Metagross is already carrying it, a Choice set might be better:
EVs: 180 HP/252 Atk/76 Spe (Adamant) or 4 HP/252 Atk/252 Spe (Jolly)
Item: Choice Band (Adamant)/Choice Scarf (Jolly)
Ability: Sand Stream
While Flygon is a good scout/revenge killer with U-turn + Choice Scarf, it doesn't have much power. IMO, Scizor is a better alternative:
EVs: 248 HP/252 Atk/8 Spe
Item: Choice Band
Running Thunder on a sand team is a big no-no, since its accuracy will be cut to 50% in sandstorm. Rotom-W lacks reliable recovery outside of Rest and Pain Split, so the latter attack is preferred over Thunder. And it seems you're using a defense-oriented set, so the preferred nature and EV spread are Calm with 248 HP/28 SAtk/232 SDef, and the item should be Leftovers. And as an alternative, if you want to keep your current spread, Thunder Wave isn't the best option in a Choice set, since you'll be locked into the move after use unless you switch out. Replace it with Trick so that Rotom-W can cripple walls and physical attackers with its Choice item. Also, replace Thunder with Thunderbolt or Hidden Power (Ice).
Since this is a competitive team, Icicle Crash is still preferred over Ice Fang on Mamoswine; you can breed female Swinub/Piloswine/Mamoswine with a male Beartic with the move. Thick Fat is also Mamo's preferred ability to give it a neutrality to Fire and a resistance to Ice. I'd put Terrakion in its place since it benefits from the 1.5x Special Defense boost in Sandstorm:
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Air Balloon/Salac Berry
Metagross should use Pursuit over Zen Headbutt, as Pursuit has more utility in catching fleeing opponents, especially Ghosts/Psychics. The EV spread should be 252 HP/96 Atk/160 SDef to help it take special hits more easily.