now... Ampharos will be good against 12/25 pokemon... so keep him
Xatu is good for Koga and Bruno, so keep him too... but watch out for the 4th member, Xatu is useless there...
Poliwrath, tick...
Slowking/Dugtrio is not of much use to you, so perhaps swap him for an Ice type...
you also need a rock type to take care of Charizard... but perhaps keep Slowking.
I suggest you get Jynx or Delibird from in-game, or get a Froslass from a trade...