Is there an easy way to grind coins
Because I think I'm just going to buy like 10 mulligans to get through the one on one putt challenge.
Easiest (aside from doing well or halfway decently in tourneys) is to play the DLC coin tourneys over and over again. You should be able to get, let's say, 5-9k out of them, so just replay them over and over, also improving your standing which nets you more coins at the end, obviously. Nintendo is really grateful that you bought the Season Pass, so they shower you with coins in those.
Edit: Ah, before I forget, you also get coin powerups aplenty in those tournaments, so use them!