get a pokemon, then you train it, get it really strong, and beat th ecrap out of all the 7th leader's pokemon. Its easy.
Water, dark and grass Pokemon
After you beat leader Norman
Beat the gym leader
get better pokemon
You should get a Duskull and evolve it [it evolves at lvl 37 to Dusklops] then get a Shuppet the evolve that [it evolves at lvl 37 too] then teach it a ghost type move or dark type move, then train and that is how to beat the 7th gym leader in Pokemon Emerald. Good luck!!
the best way to beat the 7th gym leader is with a fire type Pokemon
you get a dark Pokemon for example mightyena
with fighting type pokemon
You beat Leader Norman, then someone gives you surf.
in the fortree city
keep using phychic
You have to beat the gym leader, Pryce, in Mahogany City.