To defeat or "beat" the 7th gym leader in Pokemon sapphire there twins and they have a solrock and a lunatone to beat them I suggest about level 40-50 water types.
They are both in mossdeep city. Make sure you have 2 powerful Pokemon that can beat rock and psychic types!
go on google
Swampert.
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.
the best way to beat the 7th gym leader is with a fire type Pokemon
Water, dark and grass Pokemon
get a level 60-80 exploud that did the trick for me.
beat the second gym leader. hope this helped! : )
You have to beat the gym leader, Pryce, in Mahogany City.
go back to your fathers gym and defeat him
The gym leader in the normal gym, your character's father will give it to you after you beat him.
Use strong water or dark type Pokemon