Guess what you need at least 2 strong Pokemon because the gym leader is actually gym leaders meaning there is two that's because there twins so you have to have at least 2 strong Pokemon to verse them and win.
you need two or more Pokemon to battle them as it is a double battle.
Mossdeep City
MOSSDEEP CITY
the mossdeep cite gym has twin traners to get dive outside. and you get DIVE HM5 from steven at his house on Mossdeep City!
the seventh gym in pokemon sapphire is in mossdeep city.
In Mossdeep city I think.....
In Pokemon Sapphire, Ruby and Emerald the 7th Gym that's located in Mossdeep City has Psychic type Pokemon.
mossdeep city, to get there, surf east from lilycove also it is a double battle because they are twins Gym Leaders Tate & Liza
In Mossdeep city. On the right side of the island. (On the 7th gym island)
you battle the psychic twins, tate and lisa, in the mossdeep gym.
You must beat the MossDeep City gym to use the HM for Dive.
Go on to defeat the seventh gym in Mossdeep City, if you did not already catch Kyogre.