the seventh gym in pokemon sapphire is in mossdeep city.
Slateport City doesn't have a gym.
he is in stooptopolis city
there is a key in the Pokemon mansion... you need it to get into the gym... search the Pokemon mansion thoroughly
yes, there is. all you need to do is have every single Pokemon in your party faint except 1, or if your battling the 7th gym leaders, just battle them with 1 Pokemon in your party.
Get Kyogre..
MOSSDEEP CITY
Mossdeep City
In Pokemon Sapphire, Ruby and Emerald the 7th Gym that's located in Mossdeep City has Psychic type Pokemon.
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.
In Mossdeep city. On the right side of the island. (On the 7th gym island)
it is a dragon type Pokemon gym
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
fire gym in lavaridge
the seventh gym in pokemon sapphire is in mossdeep city.
In sapphire, the 5th gym leader is your dad, in that city after Oakdale.
i think you get to the underwater cavern after beating the 7th gym leader