The primary method used here was laid out in Abdallah's video on cash grinding. So, credit where credit's due:
Skip ahead to 6:14 and he'll tell you that 2P Training Mode is the way to go. Or you could just read my recap.
Use a pair of single joycons (or two pair if you got em) and leave player two idle. An active player can go through ARMS selection, clean out the AFK fighter, and set up the rematch in under 20 seconds. This thus earns one coin every ten seconds - one for each player in the match.
Sounds lame, but it's the fastest method described by far (unless, perhaps, you can consistently Perfect and grind high level Grand Prix - check the video for comparisons of other modes if you like).
But you know what makes it awesome? Continuously planting Helix until he
****ing stays there.
Load up Master Mummy for P1 with Phoenixes/Retorchers (or a faster set if you have it), load up Helix for P2 with anything aesthetically pleasing, and chaingrab the flubber. Five dunks later he won't bother getting back up.
Takes slightly longer than average, but I've shaved it to 16 seconds of dunk time - and trust me, the extra time is worth it.
It is so,
so worth it.