How did you guys win?
 

Any starts or just hoping for good standings?

I am not saying these tips are perfec but they worked for me:

 

Surrender and reset if you see a Golurk, it makes the challenge harde than it needs to be.

If Arcanine or Rat die during the challenge, surrender and reset.

OHKO Weavile with Flare Blitz from Arcanine or it will defeat your Rat.

OHKO Scrafty with Moonblast from Clefable.

Always deny DEF and SPATK boosts by leaving at least one pokemon on those boost-fields. (The Mega-Gengar will collect all the boosts his pokemon collected after the challenge)

Try to use moves which hit multiple targets like rockslide from Marowak, Dazzling Gleam from Musharna and Clefable and Heat Wave from Arcanine.

Try to find fields where you can hit multiple targets super-effective with these moves. (Drifloon, Honchcrow, Chandelure with Rockslide; Courgeist, Trevenant with Heatwave etc.)

Musharna is mainly there to chip multiple targets with Dazzling Gleam or protect the DEF and SPTAK boost fields.

Rat crunches.

 

Hope these tip can help you out.

Yes, thank you.
Nice tips and you also confirmed some of my thoughts. Will be easier now for sure.