Alright, so let's work through this one by one:
Toxapex-Regenerator
Black Sludge
Scald
Infestation
Recover
Toxic
This set is fine, though I'd suggest getting Haze on it instead of Infestation. Toxapex is a very passive Pokemon and because it is very weak, it invites a lot of Pokemon to come set up on it while it deals next to no damage. Haze removes the stat buffs they try to make, and so you'll still come out a winner. Alternatively, put Protect on it.
Tapu Bulu
Leftovers
Leech Seed
Horn Leech
Protect
Substitute
This is perfect.
Tapu Fini
Terrain Extender
Moonblast
Calm Mind
Scald
Taunt
Put Surf over Scald, as you're going a more offensive route and you'll want the extra power. Terrain Extender isn't that great of an item for Fini. Maybe try swapping it with Bulu's Leftovers? Surf also hits both opponents, which is awesome for doubles.
Kartana
Focus Sash
Leaf Blade
Smart Strike
Sacred Sword
Detect
This is pretty good as well. You might want to put an Assault Vest on as the item though, as I've been told that's the ideal set for VGC.
Pheromosa
Fightinium-Z
Focus Blast
Ice Beam
Speed Swap
Protect
I'm a little lost with Speed Swap here, as Pheromosa's claim to fame is it's speed and getting rid of it feels rather useless. I'm also not sure how viable Pheromosa is for doubles, as it is very frail and prone to being ganged up on.
Minior
White Herb
Shell Smash
Acrobatics
Rock Slide
U-Turn
This isn't a bad set either, though I question how viable Minior is due to how frail it is in the core form.
As you've said, a lot of your Pokemon stack weaknesses, and this is awful in doubles when you have two attackers.
Right now, your team is very weak to Torkoal + Lilligant offense, as well as to Celesteela. I also feel like your team can't deal with Pelipper Rain teams. However, you're already part way there because you've realized that weakness stacking was the problem.
So for some ideas here:
Running a Torkoal + Lilligant of your own solves most of these problems. The core is fantastic and will put in so much work, guaranteed. I know you don't want to replace them, but Pheromosa and Minior are honestly the weak links of your team. Here are the sets I'd recommend:
Torkoal @ Heat Rock
Ability: Drought
EVs: 248 HP / 248 SpA / 8 Def
Quiet Nature
- Eruption
- Protect
- Heat Wave
- Solar Beam
Lilligant @ Grassium Z
Ability: Chlorophyll
EVs: 8 HP / 248 SpA / 248 Spe
Modest nature
- Leaf Storm
- Sleep Powder
- After You
- Protect
The idea behind this core is Sun Offense. Lilligant becomes super fast on turn 1, and can use After You to make Torkoal incredibly fast, as After You makes your target move right after you. Therefore, you can fire off Sun boosted Heat Waves or Eruptions that can decimate most teams. Lilligant also packs utility with Sleep Powder, which can basically put a Pokemon out for the rest of the game, as doubles games are done in very few turns. This helps with your Celesteela problem immensely, and, as you're slower than Pelipper, you'll get your Sun up instead of their Rain. To solve more of these problems, you could also try out Drampa:
Drampa @ Life Orb
Ability: Berserk
EVs: 252 HP / 252 SpA / 4 SpD
Modest nature
- Hyper Voice
- Dragon Pulse
- Fire Blast
- Ice Beam
This covers a few more weaknesses, and provides more offensive power. It's also slow, and so benefits from Trick Room teams. Run Berserk if you want to run it with Torkoal+Lilligant, or use Cloud Nine otherwise.
Honestly, your team isn't that bad. Building teams is hard and takes practice. Now that you're aware of the weaknesses you're stacking and how much it can impact your team's success, you'll know to watch for it next time and make something even better!
