On in the

 Swampert-Mega @ Swampertite
Ability: Damp
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Earthquake
- Waterfall
- Ice Punch
- Rain Dance

Kingdra @ Dragonium Z
Ability: Swift Swim
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Draco Meteor
- Surf
- Ice Beam
- Rain Dance

Rotom-Wash @ Choice Scarf
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Thunder
- Hydro Pump
- Volt Switch
- Trick

Pelipper @ Leftovers
Ability: Drizzle
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Hurricane
- Scald
- Toxic
- Roost

Gyarados @ Sitrus Berry
Ability: Moxie
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Dance
- Waterfall
- Earthquake
- Bounce

Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Toxic
- Recover
- Scald
- Earth Power