I'm not really a competitive battler but I did use this little strategy to sweep my friend. He was not too happy about it either xD
Gengar ~ 31 IVs ~ Speed ^, Attack v nature ~ Evs ~ Max speed, max sp atk.
Moves: Hypnosis, Dream Eater, Hex, Sludge Bomb
Held: Wide Lens
Method: Hypnosis then...