The second gym leader is Gym Leader Gardenia. She uses grass type Pokemon. Cherubi is herweak pokemon so its better to use a Chimchar(if you have) or any flying pokemon to beat this pokemon. Turtwig may be easy to defeat but it have big defense power so better you must use pokemon will surely hit it away. And lastly Roselia, since it is fast and tough so better paralyz it first so that there is a chance that it can't move.
Pokemon Gym Leader Maylene is the next after you defeat Gym Leader Gardenia. It is also an easy Gym Leader, but beware of its strength. So it's better if you have a Ghost/Psychic/Flying pokemon. If you have a Pokemon that have a bubblebeam you can also use this pokemon.
Pokemon Gym Leader Crusher Wake uses water type Pokemon.As such, those who have Shinx/Luxio/Luxray or Budew/Roselia will find this gym is pretty easy. Mega Drain/Spark is all you'll need to easily win this fight.
I can consider Gym Leader Fantina is the hardest of the Gym Leader especially if you don't have any psychic pokemon. She uses Ghost type pokemon. Ghost type pokemon have the ability to levitate so a ground type pokemon is no match for it. If you have a pokemon that can use surf then use it. But be sure that it hasa high level in order to deal great damge. But be sure that you must have also a Psychic type Pokemon in order to defeat her easily.

Gym Leader Byron is Specialize in Steel Type Pokemon. A Fire type Pokemon with a Strong fire tyspe attack is suitable in this battle. Also Water and fighting pokemon can alo be ideal in this gym leader.
Except for Medicham, all her Pokemon can be taken out quite easily with a Fire, Fighting or Steel-type move like Flamethrower or Metal Claw. Medicam is weak to Crunch, so use that if possible. If not, hit it with your best moves.

Volkner is a pushover. Surf from an Empoleon will easily bowl over his Pokemon, and Ground-type moves will decimate them as well.