Dark Plume
Pokémon (16)
4x Oddish (Jungle)
1x Dark Gloom
3x Dark Vileplume (1 holo)
3x Rocket's Hitmonchan
3x Mewtwo (MP)
2x Erika's Dratini
Trainers (20)
3x Lass
3x Pokémon Breeder
2x Pokémon Trader
2x Pokémon March
2x Computer Search
2x Professor Oak
2x Misty's Wrath
2x Bill
1x Nightly Garbage Run
1x Goop Gas Attack!
Energy (24)
9x Fighting Energy
9x Psychic Energy
2x Double Colorless Energy
2x Full Heal Energy
2x Potion Energy
Strategy
I like how you have the ability to get Dark Vileplume out on turn two. Let's make turn one more beneficial, and safe for Oddish.
Pokémon
Make two Oddishes from Jungle rocket. You may not play grass energy, but your opponent doesn't know that. Add 3 Cleffa. They work great with Lass. Try to first turn Lass, then Eeeeeeek. With your big new hand, get Vileplume out.
Trainers
Drop the Bills, you don't need them. Add two Resistance Gyms, they're perfect with Mewtwo and Hitmonchan. I think you should add a Professor Elm, just cause you have Cleffa doesn't mean an Elm won't help.
Energy
Drop a fighting and a psychic. Get rid of the Potion energies, they won't do much. Full Heal will work good, though. I think that's all we need.
Revised Dark Plume
Pokémon (19)
4x Oddish (Jungle)
1x Dark Gloom
3x Dark Vileplume (1 holo)
3x Rocket's Hitmonchan
3x Cleffa
3x Mewtwo (MP)
2x Erika's Dratini
Trainers (21)
3x Lass
3x Pokémon Breeder
2x Pokémon Trader
2x Pokémon March
2x Computer Search
2x Professor Oak
2x Misty's Wrath
2x Resistance Gym
1x Nightly Garbage Run
1x Goop Gas Attack!
1x Professor Elm
Energy (20)
8x Fighting Energy
8x Psychic Energy
2x Double Colorless Energy
2x Full Heal Energy
This deck should do quite well against Hay Variants and Wiggly, and especially Rain Dance as long as you Lass.
-Ness