The best method would be to use water type pokemon being that the battle is a twin battle meaning you have to send out two pokemon instead of one.If you find you still can't win train and level up some before trying to beat the leader.
You go to Sky Pillar after the 7th gym in Emerald. There are only 8 gyms.
the 7th gym is in mossdeep city
Defeat all of the Gym leaders on Emerald. ~KKMG1
The seventh gym badge in Emerald is located in Mossdeep City.
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.
Go in. Beat the GYM. THAT SIMPLE
beat the 8th gym
mossdeep
In 7th gym
after you beat your father's gym
beat the 8th gym
Beat the gym leaders of:RustbroDewfordMauvilleLavaridgeThen return to petalburg and Norman will accept your challenge