Nothing else advancing Dungeon Defenders 2 you will gain defending medals, when happening all types of missions or maps. If you just start the game, it is advisable to play the campaign, get medals because there are missions and much more, at all times you would be winning defending medals.
After finishing the Dungeon Defenders 2 campaign, the next option is the daily quests to win more defender coins, these missions are short and easy to spend in a day, you will receive a good amount of medals. These daily missions can be repeated to enter the win 3x maps, when you finish you win 450 defending medals, it would be this mission that will generate more profit and weekly missions if you do you will receive 600 medals.