Mega Garchomp is not worth using outside of Sand teams, as Sand Force is useless otherwise. The Speed loss upon Mega Evolution does not help either. Crunch also doesn't give Garchomp noteworthy coverage; a neutral Outrage still outdamages a super effective Crunch:
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Stone Edge/Fire Fang
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lum Berry
or
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Life Orb/Rocky Helmet/Lum Berry
or
-Outrage
-Earthquake
-Stone Edge/Rock Slide
-Fire Blast/Dragon Claw
Nature: Naive/Naughty/Jolly/Adamant
EVs: 252 Atk/4 SAtk/252 Spe
Item: Choice Scarf
or
-Stealth Rock
-Dragon Tail
-Earthquake
-Toxic/Fire Blast
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Rocky Helmet
Ability: Rough Skin
Weezing is outclassed by more useful Poison Pokemon such as Mega Venusaur, Amoonguss, (which posses greater overall bulk), Scolipede (because it's stronger and faster) and Gengar (because it's stronger and faster, while still having some utility behind it):
Venusaur:
-Giga Drain
-Sludge Bomb
-Sleep Powder/Leech Seed/Earthquake/Knock Off
-Synthesis
Nature: Bold/Relaxed
EVs: 252 HP/240 Def/16 Spe
Item: Venusaurite
or
-Giga Drain
-Sludge Bomb
-Hidden Power (Fire)
-Synthesis/Sleep Powder
Nature: Modest
EVs: 232 HP/252 SAtk/4 SDef/20 Spe
Item: Venusaurite
Amoonguss:
-Spore
-Giga Drain
-Foul Play/Hidden Power (Fire)/Sludge Bomb
-Clear Smog/Stun Spore
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Black Sludge
Ability: Regenerator
Scolipede:
-Megahorn
-Spikes
-Swords Dance
-Baton Pass/Earthquake/Rock Slide
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Focus Sash
Ability: Speed Boost
or
-Megahorn
-Poison Jab/Rock Slide
-Earthquake
-Protect
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Ability: Speed Boost
Gengar:
-Shadow Ball
-Sludge Bomb
-Focus Blast/Thunderbolt
-Destiny Bond
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
or
-Substitute
-Shadow Ball
-Will-o-Wisp
-Disable/Sludge Bomb/Focus Blast/Taunt
Nature: Timid
EVs: 28 HP/148 SAtk/80 SDef/252 Spe
Item: Black Sludge
I'm honestly not fond of Magcargo in an OU setting because of its seriously crippling common weaknesses to Rock, and especially Water and Ground. Heatran or Mega Charizard X are better defensive Fire Pokemon to turn to:
Heatran:
-Lava Plume
-Protect
-Roar/Toxic/Stealth Rock
-Toxic/Will-o-Wisp
Nature: Calm
EVs: 248 HP/220 SDef/40 Spe
Item: Leftovers
Charizard X:
-Fire Punch/Earthquake
-Dragon Claw
-Will-o-Wisp
-Roost
Nature: Careful
EVs: 248 HP/148 SDef/112 Spe
Item: Charizardite X
Ability: Blaze
Mantine's 4x susceptibility to Electric moves and inability to learn Roost let it down defensively and Skarmory does what it can do better:
-Whirlwind
-Roost
-Spikes/Stealth Rock/Defog
-Brave Bird/Defog
Nature: Impish/Bold
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
I'm just putting this out there, but why Mow Rotom? Wash Rotom and even Heat Rotom can do what it can do better.
Sorry for changing most of the team around, but there are times when favorites aren't enough to succeed in competitive battling.