I wouldn't add in Vivillon as an attacker because Volcarona does what it can do better (Quiver Dance attacker):
-Quiver Dance
-Fiery Dance
-Bug Buzz
-Roost
Nature: Bold
EVs: 248 HP/192 Def/68 Spe
Item: Leftovers
Ability: Flame Body
Greninja needs Extrasensory somewhere in its moveset to deal with Mega Venusaur better (Thick Fat makes Ice Beam a neutrality on it):
-Hydro Pump
-Ice Beam
-Extrasensory
-Dark Pulse/Hidden Power (Fire/Grass)/Grass Knot/Spikes
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Focus Sash
Ability: Protean
Defog support is pretty much a necessity, given Volcarona's Stealth Rock weakness.
[email protected] can do the job:
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish/Roost/Recover
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Defog/Roost/Recover
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Excadrill makes a good teammate, as it threatens Sylveon and Aegislash, while also providing backup entry hazard removal:
-Earthquake
-Iron Head
-Rapid Spin
-Stealth Rock/Rock Slide
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Leftovers/Air Balloon
Ability: Mold Breaker
or
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe (Choice Scarf) or 120 HP/252 Atk/136 SDef (Assault Vest, Adamant only)
Item: Choice Scarf/Assault Vest
Ability: Mold Breaker
A Steel Pokemon like Scizor and Aegislash pairs well with
[email protected] for their good defensive synergy:
-Shadow Ball
-Shadow Sneak
-Pursuit/Iron Head/Sacred Sword
-King's Shield
Nature: Quiet
EVs: 244 HP/252 SAtk/12 Spe
Item: Leftovers
or
-Substitute
-Toxic
-King's Shield
-Shadow Ball
Nature: Modest
EVs: 244 HP/252 SAtk/12 Spe
Item: Leftovers
or
-Shadow Ball
-Shadow Sneak
-Flash Cannon/Sacred Sword
-Sacred Sword/Pursuit
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb/Spooky Plate/Spell Tag
or
-Swords Dance
-Shadow Sneak
-Iron Head/Shadow Claw
-Sacred Sword/Head Smash
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Life Orb
-Swords Dance
-Bullet Punch
-U-turn/Bug Bite/X-Scissor/Knock Off
-Roost
Nature: Adamant
EVs: 248 HP/136 Def/104 SDef/20 Spe
Item: Scizorite
Ability: Technician
or
-Swords Dance
-Bullet Punch
-U-turn/Bug Bite
-Knock Off/Superpower
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Scizorite
Ability: Technician
or
-U-turn
-Bullet Punch
-Superpower
-Pursuit/Knock Off
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Choice Band
Ability: Technican
or
-U-turn
-Bullet Punch/Knock Off
-Defog
-Roost
Nature: Impish
EVs: 248 HP/252 Def/8 SDef
Item: Scizorite
Ability: Technician
Try out Gliscor if you want a staller:
-Substitute
-Toxic
-Protect
-Earthquake
Nature: Impish
EVs: 244 HP/32 Def/224 Spe
Item: Toxic Orb
Ability: Poison Heal
or
-Earthquake
-Stealth Rock/Taunt
-Toxic/U-turn/Knock Off
-Protect/Roost
Nature: Impish
EVs: 244 HP/204 Def/52 Spe
Item: Toxic Orb
Ability: Poison Heal