On Route 2, during the morning you can find Hawlucha.
The reason his Hawlucha is so fast is that it's holding a synthetic seed, which boosts it's attack and applies helping hand(which is around +50% more attack), and then using the synthetic seed activates it's ability, unburden. Unburden doubles the users speed when it's held item is consumed.
My strat would be to use your own Hawlucha with a Synthetic seed.
Start by dealing with the Hariyama. Try to finish off Hariyama with something weak to flying so the Hawlucha will be more likely to be sent out next. Send in something to either paralyze or sleep the Hawlucha before sending in your own, setting up a Swords Dance and sweeping the rest of his team.