Help with a vgc team

[Marcusube]

Charizard-Mega-Y @ Charizardite Y
Ability: Drought
Level: 50
EVs: 12 HP / 76 Def / 252 SpA / 4 SpD / 164 Spe
Modest Nature
- Protect
- Solar Beam
- Heat Wave
- Overheat/Dragon Pulse/Ancient Power

There is no need to go 252 Speed EVs since after Tailwind, you out speed everything you need to with 164 EVs.

Whimsicott @ Focus Sash
Ability: Prankster
Level: 50
EVs: 100 HP / 116 Def / 72 SpA / 220 Spe
Timid Nature
- Tailwind
- Encore
- Giga Drain
- Protect

Mamoswine @ Choice Scarf
Ability: Thick Fat
Level: 50
EVs: 52 HP / 252 Atk / 52 Def / 4 SpD / 148 Spe
Adamant Nature
- Icicle Spear
- Rock Slide
- Earthquake
- Superpower

Also swap Rotom-W out for Garchomp to balance out your weaknesses and for more offensive power. 252 Atk 252 Speed with Life Orb running Protect, Earthquake, Rock Slide and Dual Chop - Jolly Nature