First check if you have 5 Pokémon aviable, if not its really impossible. You would have to fix that by redoing the old quest.
But expectign you have 5 mons, I made it this way:
See a Golurk? -> Give up. Its easier without it.
No Golurk? -> Always Flareblitz Weavile with Arcanine in 1st round if present -> Always Moonblast Scraggy with Clefable in 1st roundif present
-> Always try to hypnose Trevenant with Munna in first round if present
Try to Rockslide opponent "packs" with Cubone, means when there are 3 opponents in one field, a Rockslide is very nice because of the high flinch rate.
After the first round, try to defeat squishy Pokémon first.
Make sure there is always a Pokémon on the Spatk and Def field.
I kept Arcanine and Rattata alive in order to to Flare Blitz and Crunch the boss Gengar
Dont hesitate to give up the round if its looking bad.