How to quickly do monopoly?

I'm an ugi pls help. I did the farm stage multiple times, clearing each different group of enemies and I never got any of the platinum medals I need to buy QBZ-191. Is it just bad luck or am I missing something here?

>>715794432
why's he got a blood stain in the 2nd and 3rd panel?

>>530288774
I'm in the same spot
I have no clue how to kill him
Asked earlier in the thread but no on replied