Weather abuse is less effective in PU than in other environments, as the only auto weather setter there is Hippopotas and not too many Pokemon benefit from it.
As far as I'm concerned, Carnivine is a bad Pokemon, even by PU standards, as it's slow and frail, a bad combination to have. In fact, it's outclassed by Leafeon as a physical Grass Pokemon and by Roselia as a support Grass Pokemon. Leafeon is bulkier and faster than Carnivine, while Roselia is bulkier too, especially with Eviolite, and has a better support move pool:
Leafeon:
-Swords Dance
-Leaf Blade
-Knock Off
-Baton Pass
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Yache Berry/Occa Berry
Ability: Chlorophyll
Roselia:
-Giga Drain
-Sludge Bomb
-Spikes
-Synthesis
Nature: Calm
EVs: 252 HP/128 Def/128 SDef
Item: Eviolite
Ability: Natural Cure
Shedinja needs a ton of support to even function effectively. You need entry hazard removal to ensure that Shedinja doesn't get KOed by Stealth Rock or Spikes. I'd replace Glalie with Vibrava, as it provides Defog support and you stacked on a huge Fire weakness:
-Earthquake
-U-turn
-Defog
-Roost
Nature: Careful
EVs: 252 HP/4 Def/252 SDef
Item: Eviolite
Speaking of Shedinja, it doesn't do well with a boosting move + Baton Pass, as it needs all four of its moveslots to be effective. Even though Focus Sash is a one-time item, it's more reliable than Focus Band, as the latter item's activation rate is rather low at 11.7%. It's better to give Shedinja guaranteed one-off survival with Focus Sash than being KOed right off the bat without the Focus Band activating:
-X-Scissor/Protect
-Shadow Sneak
-Will-o-Wisp
-Baton Pass
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Focus Sash
Castform is one of the worst Pokemon you can possibly use, even in PU. Yes, it has that weather gimmick going on and has a wide move pool, but 70 stats all around is just asking for trouble. Ursaring is a more reliable, harder-hitting attacker, though be warned it will get worn down fast with Toxic/Flame Orb:
-Facade
-Close Combat
-Crunch
-Protect/Swords Dance
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Toxic Orb
Ability: Quick Feet
Leavanny should be played as a Sticky Web suicide lead to give your slower attackers an easier time against faster ones. As weird as this sounds, you're better off giving it special moves over physical ones, as the vast majority of Rock and Ground Pokemon in PU have better Defense than Special Defense:
-Sticky Web
-Leaf Storm
-Knock Off
-Electroweb/Magic Coat
Nature: Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Focus Sash
Ability: Chlorophyll
While it's true that Huntail can use Shell Smash to compensate for its lower Speed, Floatzel is the best Water Pokemon in PU, as it's very fast off the bat and has a good movepool:
-Hydro Pump
-Ice Beam
-Hidden Power (Grass)
-Surf/Switcheroo/Baton Pass
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Specs/Life Orb
Ability: Water Veil
or
-Waterfall
-Ice Punch
-Aqua Jet
-Pursuit/Return/Switcheroo
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Choice Band
Ability: Water Veil
