Hydreigon can learn Zen Headbutt move from the move tutor in black/white 2 on Lentimas town. It requires 8 blue shards to learn the move. You know when Zen Headbutt is Psychic-type move. So, yes it is super effective to cover fighting type Pokemons. If you can't earn this move well, you should learn your Hydreigon Fly or Acrobatics.
Hope this is helpful