Bulky Volcanion Team

Cobalion @ Leftovers
Ability: Justified
EVs: 8 HP / 252 Atk / 248 Spe
Jolly Nature
- Close Combat
- Iron Head
- Stealth Rock
- Volt Switch

Volcanion @ Leftovers
Ability: Water Absorb
EVs: 248 HP / 252 Def / 4 SpA / 4 Spe
Bold Nature
IVs: 0 Atk
- Steam Eruption
- Flamethrower
- Defog
- Toxic

Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Earthquake
- Knock Off
- Swords Dance
- Rock Polish

Sylveon @ Leftovers
Ability: Pixilate
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
IVs: 0 Atk
- Hyper Voice
- Heal Bell
- Wish
- Calm Mind

Tsareena @ Assault Vest
Ability: Queenly Majesty
EVs: 100 HP / 252 Atk / 156 Spe
Adamant Nature
- Trop Kick
- U-turn
- High Jump Kick
- Rapid Spin

Banette-Mega @ Banettite
Ability: Frisk
EVs: 124 HP / 252 Atk / 132 Spe
Adamant Nature
- Shadow Claw
- Destiny Bond
- Will-O-Wisp
- Taunt

I'll comment below so you can just copypaste the team without any issues.

Cobalion: even uninvested has ok physical bulk to switch in on dark physical attacks for Banette. Safe switch-in for Mega-Beedrill but must take care with Drill Run

Volcanion: tanks relatively well a lot of physical mons and has a chance to burn them. Other water absorb mons will get toxified. It's surprisingly good, survives at least one STAB earthquake/stone edge unless it crits and OHKO Mega-Aero and common Gliscors. Scares the shit out of Scizors while you can fish for burns instead if the opponent's team has many physical mons. Defog also makes Steam Eruptiom indirectly more accurate. You will have 2 or 3 tries to ruin Azu's day burning him if full hp.

+1 252 Atk Aerodactyl-Mega Stone Edge vs. 248 HP / 252+ Def Volcanion: 320-378 (88.1 - 104.1%) -- 25% chance to OHKO after Leftovers recovery
4 SpA Volcanion Steam Eruption vs. 0 HP / 4 SpD Aerodactyl-Mega: 308-366 (102.3 - 121.5%) -- guaranteed OHKO

Gliscor: These evs are like that on purpose. Knock off punishes enemy's own Gliscor switching in, you can setup on pretty much anything not too offensive. You survive a hidden power ice from Mega-Manectric, I like to SD first, then while they think they will kill me with HP I RP, OKHO the manectric and I'm now a threat. It takes 2hko from max attack Ice Fang Gliscor, unless they SD first. I'm bad and I could get this mon to +6 atk +2 speed surprisingly many times because he simply won't die easily to not too offensive mons, even without roost.

Sylveon: Laughs at Hydreygon for days, it's your main switch in for special attacks, except for Nidoking and some steel types. Doesn't have a good response for Calm Mind + Psyshock Latias if she setups first. You can bet against some mons that have poison moves as coverage but don't have speed investment and die to Hyper Voice.

Tsareena: Outspeeds Azumarill's Play Rough, doesn't take Aqua Jet. Opponents switching in may have its attack dropped. You want to switch to Volcanion predicting rocks so he won't take unecessary damage, them you deal with the mon by attacking/burning or toxifying him. If it's too late, Tsareena is also an option. If one of them dies the other can cover the role.

Banette: Taunt mons that you predict will try to setup hazards/boosts/common toxic/wow users. Destiny bond to scare them. Shadow Claw in the following turn guarantees that if your opponent attacks you, he dies. Priority WoW is always nice. Outspeeds Azu so it can burn/destiny bond him in case you want to preserve other mons to deal with the enemy team.