Roxanne: Have a Mudkip or Marshtomp that knows Water Gun and it will deal a lot of damage to her Pokémon. Brawly: Capture a Sableye in the cave next to Dewford, and he will not be able to make a hit on you. This battle will take a little time, though. Wattson: Have a Marshtomp that knows Mud Shot, and he is practically rendered useless, except his Magneton can be a little annoying with his Sonicboom. Flannery: Have a Swampert that knows Muddy Water, and keep on using it and she will be finished in little time. Norman: Have a Hariyama or Lairon, Hariyama that knows Low Kick, Lairon for defense, and keep on using it until he is finished. This could take some time, though. Winona: Go to Lilycove and buy Blizzard and Thunder and teach it to Swampert and any Electric Type, and use Blizzard on every Pokémon except for Skarmory. Use Thunder on Skarmory. Tate & Liza: Have a high level Sharpedo, at least level 40 or 45, that knows Crunch, and keep on using that and Surf with Swampert and they will be finished pretty easily. Juan: Have a high level Electric and one high level Grass Pokémon, Raichu and Sceptile is recommended, that knows Thunder or Thunderbolt and any Grass move, and keep on using Thunder or Thunderbolt until he brings up Whiscash, and switch to the Grass Pokemon and use that Grass Pokemon until he chooses Gyarados, and use Thunderbolt and he will be finished easily.
