Auto Fight farming.

Once you get above a certain level (and I'm afraid I'm not sure what it is) you unlock the Auto Fight facility in dungeons.  Auto Fight can be turned on and off using the button at the bottom-right of the screen.  

In a Normal level dungeon, Auto Fight basically keeps replaying the chosen mission until you stop it, you are defeated, you run out of AP or you run out of space for cards which drop.

Auto Fight can be used very effectively to farm for exp, puzzle pieces, fodder cards and even Evolution armour sets.  Just follow these simple steps:

* Make sure you have plenty of space for dropped cards (you may want to use Storage or purchase additional slots)
* Choose a Normal dungeon and a mission that your team can win reliably on its own.  Helpers will be assigned automatically in the order they appear in the queue so you can't depend on them for help.  Your team, in the formation you choose, must be able to win even if the game assigns a complete lemon as a helper!
* Start the mission as you usually would, turn on Auto Fight and leave it to it.  Come back in a while to fuse cards, etc.

It's a good way of increasing your level if you have AP to spare - just give it a fresh refill to play with and it'll carry on fighting without you having to pay attention to it. Like when you're at work....