Preparing for battle
First thing you should do: Level up your Clefable, Onix, and Dewgong to level 50-60
Second thing: Don't have your Pokemon hold the Carobs and Protein, feed it to them.
Third thing: If possible, give all of your Pokemon items.
Fourth: Buy Revives and Potions
Lead your battle with Clefable.
If you are leading with Clefable, use it's Fighting type moves to take down Dewgong and Cloyster, then Switch to Zapdos for Slowbro. After you defeat Slowbro, Switch to Charizard and use Fire type moves to defeat Jynx, then switch back to Clefable and use Fighting type moves to defeat Lapras.
Once again, lead your battle with Clefable and take out Bruno's Onix with Fighting type moves. Next, switch to Charizard and use Flying type moves to take out Hitmonchan and Hitmonlee. Now switch back to Clefable and repeat the first part for Bruno's other Onix. For Machamp, do the same as you did for Hitmonlee and Hitmonchan.
Agatha will be tough for you since you don't have any Poison or Dark type moves. Just try not to use Normal and Fighting type moves. After her first Gengar, switch to Zapdos and use Electric attacks to take out Golbat. After you defeat her Haunter, switch to Onix and use Earthquake to take out Arbok. Now defeat Gengar.
Lead with Zapdos and use your Electric attacks to defeat Lance's Gyarados. Next, switch to Dewgong and take out his two Dragonairs, his Aerodactyl, and Dragonite with ice type moves.
Defeating the Champion, Blue
Lead with Dewgong and use Ice type moves to defeat Pigeot. Next switch to Zapdo, use Thunder Wave to paralyze his Alakazam and attack it. Now switch to Clefable and use Fighting type moves to defeat his Rhyhorn. For Arcanine, use Onix's Earthquake to take it down. For Exeggutor, use Charizard's Flying or Fire type moves to take it down. Last is Blastoise, use Zapdos's Electric type attacks to take it down!
Hope I helped!