Hi Jimbo!
If you don't mind grinding with mono earth, stick with it until you meet your other goals. I do suggest you try grinding mages with fully upped earth deck. If you feel bored, build other grinder decks for other AI targets. There's so many viable decks for grinding at every AI target. I also used an upped, Mono Earth deck with stone skin to grind early on. I found the streak bonus, reliability, and less clicking compared to other decks was worth it. I think this is what I used to grind Mages:
For champs, I used so many different decks I can't remember one being the grinder deck. And that's the point... why not try out many viable decks, have fun, and still earn a good bit of coin along the way. Lots of grinder deck ideas in this
forum section.
I got most of the cards I need after 1-2 months (even upped), so we don't really need to grind for long. That's really nice because it lets you focus on deckbuilding for PVE and PVP pretty quick. Don't forget you get nice coin from coliseum every day, and coin from submitting arena decks, too.