Your team is pretty weak to Tyranitar. Scizor is OHKOd by Fire Punch/Fire Blast and the faster varients are able to outspeed and KO before you can launch Bug Bite. And Bullet Punch only 2HKOs Tyranitar. Dragonite is taken down by Ice Punch or Stone Edge, and the aforementioned faster Tyranitar variants are able to 2HKO Vaporeon with Stone Edge or Thunderpunch before you can 2HKO with Surf. Tyranitar with Superpower are able to easily take down your Tyranitar. Meloetta is just easily destroyed. And even Mew is only able to 2HKO Tyranitar with Drain Punch, while being KOd in return.
So obviously this problem needs fixed. Seeing as Meloetta and Mew are pretty much the same Pokemon in this case, we'll be replacing Mew. Now the replacement is going to need to be a Fighting type. Use either Conkeldurr or Virizion. Conkeldurr can easily set up a Bulk Up as Tyranitar switches out while Virizion is able to set up a Swords Dance. Virizion's only problem is going to be Fire Punch, Fire Blast it can take. I would prefer Conkeldurr in this spot.
You also have a problem with Bulky Waters, such as Jellicent. Now the best way to fix this is probably going to be to add in either a Calm Minder or an incredibly hard hitting Super Effective Special Attacker (Special due to Will-o-wisp/Scald). Calm Mind you can kill two birds with one stone and add in a Calm Mind Virizion in place of Mew. Or you can replace Tyranitar with a Thundurus-T. I would prefer Thundurus-T.
Now for some fixes to the rest of your team.
On Scizor, DOuble Hit is not very useful. Plenty of foes in OU resist it. And not many use Sashes and while a couple use Sturdy, Double Hit isn't going to be hitting hard enough to break through a Donphan. As such, it would be best to replace it with Superpower, which can hit Steel type foes for harder. I also would suggest swapping those HP EVs for Speed, as I find Speed more helpful for a Scizor to hit foes such as Tyranitar that haven't been given any Speed EVs.
For Dragonite, go with the standard EV spread of 242 HP / 44 Atk / 212 Spe
It works best in my experience.
For Vaporeon, go with the set of Wish, Scald, Ice Beam, Protect with an EV spread of 248 HP / 252 Def or SpDef / 8 whatever. Acid Armor should only be in use with Baton Pass, which doesn't really benefit this team much. And Wish provides superior healing to Aqua RIng. The EV spread gives it stats worthy of the title of bulky water.
FOr Meloetta, I would suggest going with an EV spread of 252 SpAtk / 4 SpDef / 252 Spd. Meloetta is actually in a speed tier where it manages to outspeed most of its threats that would otherwise take it down (as long as they have unboosted Speed). So a Calm Minder is actually pretty dangerous. You could also consider running Thunder over Thunderbolt. Politoed is used (pardon me if this is incorrect, but this is just what I remember) on about 15% of teams, so Thunder will get 100% accuracy a decent amount of the time. But even without Rain, Thunder still has the same accuracy and power as a Focus Blast. But the main attraction to using Thunder is the 60% paralysis rate, which allows it to slow down Scarfers and boosters.
Scizor (M) @ Life Orb
Trait: Technician
EVs: 252 Atk / 4 Def / 252 Spd
Adamant Nature (+Atk, -SAtk)
- Swords Dance
- Bullet Punch
- Bug Bite
- Superpower
Dragonite (F) @ Leftovers
Trait: Multiscale
EVs: 252 HP / 44 Atk / 212 Spd
Adamant Nature (+Atk, -SAtk)
- Dragon Dance
- Dragon Claw
- Roost
- Fire Punch
Vaporeon (F) @ Leftovers
Trait: Water Absorb
EVs: 248 HP / 8 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Scald
- Ice Beam
- Wish
- Protect
Thundurus-T (M) @ Choice Scarf
Trait: Volt Absorb
EVs: 4 Def / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
- Thunderbolt
- Focus Blast
- Grass Knot
- Hidden Power [Ice]
Meloetta @ Leftovers
Trait: Serene Grace
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature (+Spd, -Atk)
- Hyper Voice
- Psyshock
- Thunder
- Calm Mind
Conkeldurr (M) @ Leftovers
Trait: Guts
EVs: 248 HP / 252 Atk / 8 Def
Adamant Nature (+Atk, -SAtk)
- Bulk Up
- Drain Punch
- Mach Punch
- Payback
