Easy way to beat all Gym Leaders - Guide for Pokemon Emerald

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.