I see people who still haven't got this challenge completed, so I offer my tips to help you save a golden hammer or two.

Build a stage with platforms directly spikes, set a coin match up for maybe 10 minutes or so, then drop the characters down onto the spikes so they keep bouncing straight up and down between the spikes and platforms.

I managed to get roughly 22,000 coins in 15 minutes using two Sheiks because she seemed to fall faster than Marth.