What level is your party? In BW, Ghetsis' Pokemon are at 52, so make sure you're a few levels above that. Confagrigus is really annoying with Toxic, so I suggest use a Steel Pokemon, a dark type Pokemon like Zoroark, or use Toxic yourself on Confagrigus.
Bouffalant - use a fighting Pokemon like Mienshao/ Scrafty and one STAB Hi-Jump Kick will sort it out. If you're cautious, add a Fighting Gem for a sure 1HKO.
Seismitoad - Grass move. Be careful of Sludge Bomb on a Grass type, and it will outspeed you with Rain Dance and swift swim. Roserade or Amoongus will be best because of the Grass/ Poison typing.
Bisharp - use your fighting type again; 3 out of 4 of its moves are x1/2 against a fighting type.
Hydreigon - use a bulky water type (if you got Oshawott as your starter, that will do nicely) with Ice Beam, or your Fighting type again.
Eelektross has no weaknesses, so use I would use your highest level Pokemon with its highest STAB move to deal with it. Eelektross is pretty frail, really.