Slurpuff @ Weakness Policy/Sitrus Berry/Kebia Berry
Unburden
EVs: 252 SpAtk / 252 HP / 4 SpDef
Modest Nature
- Energy Ball
- Psychic
- Calm Mind
- Cotton Guard
Slurpuff covers both of the weaknesses of VileToad, and can set up a Calm Mind beforehand, enabling it to OHKO Seismitoad with an Energy Ball:
Atk Seismitoad Earthquake vs. 252 HP / 0 Def Slurpuff: 118-139 (32 - 37.7%) -- 92% chance to 3HKO
+1 252+ SpA Slurpuff Energy Ball vs. 252 HP / 252+ SpD Seismitoad: 420-496 (101.4 - 119.8%) -- guaranteed OHKO
It also outspeeds both of the Pokemon naturally.
If Vileplume dares to use a Sludge Bomb, Weakness Policy and Unburden will kick up - up against a +3 Psychic with an already weakened due to the original +1 Psychic as Slurpuff outspeeds.
Alternatively could use a Sitrus Berry to recover from the Sludge Bomb, and still activate Unburden.
Or even use a Kebia berry to reduce Sludge Bomb's damage.
+1 252+ SpA Slurpuff Psychic vs. 248 HP / 44 SpD Vileplume: 252-298 (71.3 - 84.4%) -- guaranteed 2HKO after Black Sludge recovery
0+ SpA Vileplume Sludge Bomb vs. +1 252 HP / 0 SpD Slurpuff: 198-234 (53.8 - 63.5%) -- guaranteed 2HKO
If you combine with the Earthquake damage by Seismitoad = (85.8% - 101.2%) -- 8.4% to KO.
With Kebia Berry:
0+ SpA Vileplume Sludge Bomb vs. +1 252 HP / 0 SpD Kebia Berry Slurpuff: 99-117 (26.9 - 31.7%) -- guaranteed 4HKO
Which gives no chance of a combined KO.
This set also has the element of surprise, people could expect an offensive Slurpuff and swap the Seismitoad in anyway, enabling a free turn to set up the Calm Mind.