Hydreigon has access to 2 flying type moves begin Acrobatics and Fly It has no access to other moves which are super-effective against Fighting types, but generally when facing one Hydreigon would either 1. switch out or 2. use Draco Meteor xD Hyderigon's base 125 SpA is generally enough to cover it against fighting types with STAB Draco Meteor providing he hits first of course =)
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
