my Pokemon are lucario lvl 55, houndoom lvl 48, staryu lvl 47, haxorus lvl 54, golurk lvl 54, magnezone lvl 54. also move sets are helpful too. thx
Go onto Route 23, and run near a patch of grass for shaking grass & train against Audinos.
Give your Pokemon the Lucky Egg you got from Juniper to boost exp. as well.
Those Audino are 48-51, so they should be the right level for you.
Staryu should really be Starmie by now, and you don't have a solid counter for Marshall. I suggest you evolve Staryu and give it a Psychic move (Psyshock/ Psychic would be best).