Well idk how it goes for you, but when ever I farm, those 3-4 fat majins spawn and always there is 1-2 silver coins drop plus mystery items that they leave behind and those are silver coins too, well most of times.
Still like I said, it shouldn't be easy, after all this is the world boss event and you get world boss rewards.
Btw some of coins u get in mystery item,doesent stack together.