Looks decent, here's what I would say. If you lose to bulky waters, or other stallers, you need more setup, and you only have Garchomp now, and he doesn't really fit on a rain team anyway. Here is what I would reccomend:
Pelipper: Looks fine. You could consider using Scald over Weather Ball, as Pelipper is more of a support than an attacker.
Barraskewda: Have you considered running Choice Band instead of Life Orb? Barraskewda is a good choiced option, just replace Aqua Jet with Flip Turn, you're fast enough to not need priority.
Ferrothorn: Ferrothorn is good in rain. I would use Leftovers instead of Rocky Helmet, as reliable recovery is better then more chip damage in this case. Also, use Power Whip over Gyro Ball, it's better. Consider using Protect over one of your moves, for Leftovers and Leech Seed. Maybe Toxic as well.
Greninja: If you want to use a Greninja, don't use Sludge Wave, Ice Beam already covers Grass, and using a Move just to fight Fairies is a waste of a moveslot on him. Use Surf instead of Sludge Wave, because Water Shuriken is low power, even with Loaded Dice. I noticed you don't have a Z-Move user, or a Mega, even though this is Natdex. Ash-Greninja is a great Z-Move user, because he can use Hydro Vortex or Black Hole Eclipse to get a KO and activate Battle Bond. Use Waterium Z or Darkium Z for your item.
Replace Zapdos and Garchomp, There are better Electric wallbreakers than offensive Zapdos, and better Grounds than Garchomp.
I would replace Zapdos with this:
Xurkitree @ Choice Scarf
Ability: Beast Boost
Tera Type: Electric
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
IVs: 0 Atk
- Thunder
- Volt Switch
- Dazzling Gleam
- Energy Ball/Signal Beam
Or this:
Thundurus-Therian (M) @ Leftovers
Ability: Volt Absorb
Tera Type: Electric
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Thunder
- Nasty Plot
- Focus Blast/Psychic/Dark Pulse/Flash Cannon/Volt Switch/Literally Anything Special
- Weather Ball
Either one would be fine.
And last, but not least, a Mega. Specifically, a Ground type Mega. The staple of Natdex Rain teams everywhere...
Mega Swampert!
This guy is literally what you need for this team. He's does what your Garchomp is supposed to do, but better.
Here's a standard set:
Swampert-Mega @ Swampertite
Ability: Damp
Tera Type: Water
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Bulk Up
- Waterfall
- Earthquake
- Ice Punch/Knock Off/Rock Slide/Superpower
This guy will rip through anything and everything.
+1 252+ Atk Swampert-Mega Earthquake vs. 248 HP / 136+ Def Toxapex: 344-408 (113.5 - 134.6%) -- guaranteed OHKO
Hope I helped!