In the name of convoluted plans I want to use just because they'd be so damn silly:

Left slot
Miltank, +Def -attack nature with leftovers
Choice of ability

Gyro Ball
Milk Drink
Psych Up
Return

Center
Clefable, +Sp.atk -attack nature with weakness policy
Magic guard

Cosmic Power
Stored Power
Moonblast/Dazzling Gleam
Moonlight

Right slot
Gourgeist (Size is up to you, but it MUST outspeed Clefable [not hard]), +speed -Sp. attack with no item
Pick up

Trick
Phantom Drive/Shadow Sneak
Light Screen
Protect
---

THE GAME PLAN

Turn one:
Gourgiest uses protect
Miltank gyro balls Clefable for very minor super effective damage, activating the weakness policy, boosting attack and sp. attack two stages for clefable
Clefable uses Cosmic power to raise Defense and Sp. Defense +1 each
Gourgiest's pick up salvages the weakness policy

Stored power is a base 140 power psychic move by the end of turn one

[If at any point either clefable or miltank are badly damaged, milk drink/moonlight and use the turn to have gourgiest put the other shield up]

Turn 2

Gourgiest uses trick to give clefable back the weakness policy it recovered at the end of the last turn
Miltank hits with an even weaker gyroball
The weakness policy activates again, giving clefable a total +4/+4 Atk/Sp.Atk
Clefable uses cosmic power to have +2/+2 in Def/Sp.Def
Gourgiest's pick up salvages the weakness policy

Stored power is a base 260 power psychic move by the end of turn two

Turn 3

Proceeds the same as turn 2, Clefable is now +6/+3/+6/+3 Atk/Def/Sp.Atk/Sp.Def and Stored power is a move with 380 base power

Turn 4 and onward

Psych up Miltank

Heal and sweep as needed, gourgiest can move around items (try to grab an opponent's life ball and give it to clefable to make things even more ridiculous) if it survived this long without heals.

Laugh at the opponent for not hazing after turn 1.

If you want to be a huge asshole, replace Cosmic Power with Minimize