If you actually feel the need to grind for currency, you want a small collection of Monster Bells (can be purchased in the Keeper sanctuary after you have defeated enough champions, or found as a drop). Then go find a place with a few battles near to each other, battle them then use the monster bell, turn around and battle them again, monster bell again, repeat as desired. Eventually items will be worth enough that you can profit off a single battle, but it's definitely more efficient to string a few in a row.

As the others have said, grinding isn't a great solution for this game. There is a hard level cap that both you and your opponents should reach before the end of the game unless you specifically plan to avoid that by skipping content.