How to defeat Ghetsis in Pokemon Black?

Okay, to be honest, ad if you can, train them a little higher to lv. 53-56, that'll help you a little bit, but as for that Hydregon of his, I would use like a fast pokemon that are the following types: Fighting, ice, or dragon. If you could train your Cubchoo into a Beartic, that should take care of Hydregon. But in this case, best way to take it down is to use your best moves on it. Like Reshriam out first and use Fusion flare, and if it faints, then bring out Samurott or Darmanitan and keep whaling on it till its down. But yeah, you need to train some more. hope i helped.