Charizard Y:
I know RNG gives cancer all the time but try getting Fire Blast instead of Flamethrower, it turns lots of 2HKOs into OHKO. Also, Try getting HP Ice if possible so you can get surprise kills on certain dragons.
Toxapex:
As a tank, I recommend Chansey and Ferrothorn instead. Toxapex is very passive outsides using Scald while Chansey can constantly do dmg with Seismic Toss and Ferrothorn can chip opponent away with Leech Seed or break sashes using Stealth Rock.
Mimikyu:
90 Base attack is hard to be mediated by Swords Dance. Try on Z crystal or Life Orb. Destiny Bond is also an interesting option because everyone is so slow in Gen 7.
My seemingly gimmicky team with 200+ win streaks in Battle Tree:
Pheromosa @ Choice Scarf
Ability: Beast Boost
EVs: 252 Atk / 4 SpA / 252 Spe
Hasty Nature
- High Jump Kick
- U-turn
- Ice Beam
- Poison Jab
Chansey @ Eviolite
Ability: Natural Cure
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Toxic
- Stealth Rock
- Soft-Boiled
- Seismic Toss
Excadrill @ Focus Sash
Ability: Mold Breaker
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- Iron Head
- Rock Slide
- Swords Dance
Hope this helps.
