I ran into a guy running this setup, and it was so annoying that I couldn't battle again until I bred one for myself and pissed off a few people.
The Little Brother Togekiss (Togekiss) @ Choice Scarf
Ability: Serene Grace
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Thunder Wave
- Nasty Plot
- Air Slash
- Roost
They can't attack 25% of time while paralyzed, and as long as you're first, they flinch 60% of the time. If there's anything that can out-speed you while paralyzed, and you're holding a Choice Scarf, Nasty Plot helps. Mathematically, they should only be able to hit you 30% of the time (so, every 3-4 turns), and hopefully they'll be paralyzed if you need to Roost.
(In all honesty, I added Roost myself, I don't know the 4th move of the guy I was fighting. He paralyzed my guys off the bat, and I managed to hit him once during 1 of his 2 Nasty Plots, but beyond that he OHKO'd my Pheromosa, and just picked away at the other 2. Luck was clearly on his side that match, and I haven't been able to give a whooping like the one I got, but it's still fun to picture your opponent smashing their DS, after flinching 3 or 4 times consecutively.