Gyarados (M) @ Toxic Orb or Flame Orb if using Facade / Life Orb if using Ice Fang
EVs: 252 Atk, 128 Spd, 96 HP, 36 Def / S.Def
Nature: Adamant ( +Atk, -S.Atk )
Aqua Tail / Waterfall if you care about Accuracy
Facade / Ice Fang If you don't like your Pokemon to be Poisoned or Burned
Dragon Dance raises Attack and Speed which is good for Gyarados
Earthquake is best for Electric, Fire, Rock, Poison, Steel
Aqua Tail / Waterfall is STAB
Facade is 140 Power with the Pokemon holding Toxic or Flame Orb
Ice Fang is good for taking out the Grass, Flying, Ground, Dragon Types blocking the way.
Overall he is your Clean Sweeper of your Party.