Cazak didn't use weak poke'mon. He just used the poke'mon he newly caught just like Ash. Also Cazak used one of his most powerful poke'mon(Gyrados) against Dorien, that's how it seemed easy. Before his Mantine lost to that of Dorien's. If I had Ash use Charizard against Gracia, it would've been a walk in the park for him too.
Yeah I will continue, but Ash isn't weak. It's just like other animes, like Beyblade for eg. Tyson is the main hero, but Kai always seems to be the strongest in the series. It's only in the end did Tyson emerge to be no.1
Here too Ash did beat Cazak in the Sinnoh League.