You need to beat the leader Norman (Your father in the game), the 5th gym leader.
The gym in Petalburg.
You must beat the MossDeep City gym to use the HM for Dive.
The sixth gym AKA the fortree city gym specializes in Flying Type Pokemon.
you have to find steven get a Devon scope and use it on the invisible keckleon in front of the gym
The sixth gym let's you use fly. It's a flying type gym so bring rock and electric pokemons.
dark type
I believe that is after you've beaten the 4th Gym in Volcanoridge Town.
Easy you go see steven to get the HM Dive then use it to go to Sootopolis City where your last gym is.
get a level 60-80 exploud that did the trick for me.
bug, water, dark, ghost
This gym uses mainly fire type Pokemon. In order to beat this gym you must use water type Pokemon lvl's 23 and up in order to defeat this gym leader.
Use Fighting type like Macop(LV 36Best Shot)