Hydreigon is quite easy because I have an electric-type pokemon in my team... Rather, the hardest for me (and until now I can't beat Ghetsis because of this bastard) is the blue toad that makes rain to UP his speed and use muddy water. All the pokemon in my team has a weakness against water and ground... So I am training a new grass-type pokemon just to beat the s*** out of this guy...
I used Hi Jump Kick with my Scrafty. Bye bye Hydreigon! :D