Hello there
First thing don't use Croagunk or evolve it into Toxicroak (level 37 i think)
Get Granbull intimidate because it can be your best mon here..
Pachirisu can abuse T-Wave heal spam along with volt absorb..
Golem can use nature power gear grind along with sturdy (if you have that)
Togedemaru can use electric terrain and nuzzle to change his field..
Obviously getting your pokemons to level 40 will also help and your best places to grind are beneath the grand staircase (last floor unown) and below shades gym..
Healing items (ice creams and lemonades) are always your best friends..