Manaphy @ Waterium Z
Ability: Hydration
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Scald
- Tail Glow
- Rain Dance
- Rest/Psychic/Energy Ball/Ice Beam

Tapu Koko @ Leftovers/Life Orb
Ability: Electric Surge
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Thunder/Thunderbolt
- Dazzling Gleam
- Volt Switch
- Roost/Hidden Power Ice

Scizor-Mega @ Scizorite
Ability: Technician
EVs: 252 HP / 68 Def / 188 SpD OR 252 HP / 252 Atk / 4 Spe
Careful Nature / Adamant Nature
- Bullet Punch
- Pursuit / Superpower / U-Turn
- Swords Dance / Knock Off / U-Turn
- Roost

A basic Rain core, that doesn't rely on Rain (but can benefit from Z-Rain from Manaphy being up.)

Waterium-Z is absurdly stupid, and gives Manaphy a way to beat HO, Balance, and Stall all in one go. Scald hits everything hard under the rain, and with a Tail Glow or two 3HKOs become OHKOs and OHKOs become 5x over OHKOs. Tail Glow buffs Manaphy's Special Attack to stupidly strong levels, and with Z-Rain Dance boosting Speed very little can stop you bar Unaware SpDef Clefable, TWave Chansey, and other pink blob mons. The final slot is filler, with Rest providing free healing and a cured Sleep status should Rain still be up at the end of the turn, Psychic hitting Venusaur, Energy Ball hitting bulky Waters, and Ice Beam hitting Dragons and doing neutral damage to Venusaur.

Tapu Koko is a great abuser of Rain, with STAB 100% accurate Thunders to work with. Should you not want to rely on Thunder too much, Thunderbolt can be run instead. Koko breaks through most bulky Waters for Manaphy should it not be running Energy Ball, and usually can outspeed and 2HKO anything else that threatens it with Thunderbolt, Dazzling Gleam, and Volt Switch. I gave options on the item and the final slot as Leftovers + Roost makes Koko a very hard zit to pop, and increases its survivability tenfold, while Life Orb and Hidden Power Ice make it more of an offensive powerhouse.

Scizor is a wincon with either EV spread, one of which avoids OHKOs from almost all Hidden Power Fire users such as (a really rare set) Modest max Spatk Life Orb Lele and tanks +2 Thunderbolts from Scarf Timid Xurkitree, should it somehow get to that point (why would you?!). Bullet Punch is a must, as priority is useful even WITH Lele roaming around right now, and the 2nd slot can be Pursuit for Metagross, Lele, and hitting Steels neutrally, Superpower for hitting bulky Steels for SE damage, and U-Turn for being a Pivot core with Tapu Koko. The 3rd slot is best off being SD, but if you run Superpower you could just go for U-Turn for pivoting + coverage or Knock Off if running U-Turn in the 2nd slot for better damage versus Metagross/Jirachi/bulky ghosts and removing items. Last slot is Roost just to heal HP back up.