Farm like crazy and you will get rewarded. While it can be time consuming if you get a good system in place you can minimise the time you spend doing it and maximise the amount you can farm. Pretty much most items can be farmed, farming medals and farming resources should be your real concern at the start but as you move on in the game you must start large scale gem farming to get very high gear. Its relatively easy to get things if you know where to look. You can farm a few other things that are not strictly necessary such as prestige or Michelangelo's scripts, and there is a lot of different farming methods suited to farming different things.