For Skarmony I recommend this set.
Skarmony @ Rocky Helmet
Ability : Sturdy
Nature : Impish ( Def+ Spa- )
EVs : 252 Hp / 64 Def / 192 SpD
- Toxic
- Spikes
- Whirlwind
- Taunt
I think this set is the best one. The EV spread balances Defence and Special Defence, and also Max Hp, giving it more bulk. If you want it to be Defensive, then max out its Defence. Taunt prevents Hazards being set up and also to prevent Skarmony from being Taunted. If you get taunted, then switch out since Skarmony will Struggle. It works great as your lead and Hazard setter.
For Cloyster replace it with this.
Slowbro @ Leftovers
Ability : Regenerator
Nature : Calm ( SpD+ Atk- )
EVs : 252 Hp / 128 Def / 128 SpD
- Fire Blast
- Slack Off
- Scald
- Calm Mind
CM set. Calm Mind gives boost to his SpD, making him a Special Tank, also boosting Scald and Fire Blast. Scald is good move with great accuracy and chance to burn. Fire Blast deals with mons like Ferrothorn. Regenerator is great for a tank like him.
For Magnezone, change the IVs to 0 Spe.
For Metagross, I recommend this.
Metagross @ Metagrossite
Ability : Clear Body > Tough Claws
Nature : Adamant ( Atk+ Spa- )
EVs : 252 Hp / 252 Atk / 4 Spe
- Rock Polish
- Meteor Mash
- Zen-Headbutt
- Earthquake
Mega Metagross is fast enough and no need Max Speed since it has Rock Polish. Rock Polish can allow him to outspeed Scarfers and fast attackers.
