What is the best way to grind coins??
Don't get banned! :) Kidding!
1) If you haven't done the Legends solos, you can build two full Legends that, once sold, yield more money than all the other coin sources available in the game.
2) There remain MANY ways to work the Auction House, but it requires understanding of micro-markets within the AH that have consistent patterns. With the Unstoppable promo release, getting good at understanding trading ranges for 91s/88s/85s/82s (guessing on the values of the 85s/82s) can help you build bank.
3) The best way to build coin is to not spend coins on playable cards. Use BNDs for solos and challenges and you'll be fine until you have a sizable bank.
4) The price range for 70-77 OVRs are exceedingly narrow, which means that grabbing any of the them less than, say, 600 coins and selling for ~1,200 will yield consistent profits.
Just some ideas - good luck!
I have found the easiest (maybe not fastest) way is through Solo Seasons...can easily grind a SB win (in the easy one) every few days (could do it much quicker I would bet, but I don't play that long).
this year its just playing the game and being good at it. Most feared neted me 2m with the brock purdy and carlton davis through draft, 87 and 90 auctionable through fear store (obtained through draft wins)
also get about 200k a week from champs and sometimes more if packs are hot, 16 wins gets you 100k, a champ 100k QS player and then two touchdown packs and gauntlet has great rewards too.