You need to use weak attacks. Once he runs, health stays the same.

Getpokemon radar to get the therian form its awsome

After defeating drayden you go to route 7. It should be rainy there. Then go to a house
after you go through the balance beams. Then go near a reddish house and a wise girl will
let you come inside. After you go outside you will see tornadus fly to a hill close by.
Then go to one of the electric boards in a route changer and then check the bulletin board
and it will tell you where a strong or violent storm is happening. Go to that place and go
in the grass if it is raining. And i used an ultra ball to catch him.