This is best done in online co op with the Den of the Dead Depths map. It will require 400 medals to reach rank 30. Depending on skill, efficiency, level, etc, I estimate anywhere from 10-12 hours to hit rank 30. Your progress for the medals can be viewed in the "pause" menu by tabbing over to medals. The map can be found right after defeating Oliver Collins. If you're teleporting there, it's Ruined City Underground to Outer Cross Roads. Shang will appear close to the mistle (you can speak with him for the Den of Darkness map), but immediately to his left if you run down the alley and take a left, the Den of the Dead map will be a shiny mark laying on the ground in plain sight. The process is to summon your partner at the mistle in the depths, and run through all 3 bosses. 2 are mini bosses, but you will both receive medals for defeating them. The boss behind the door is Oliver Collins, again. As soon as you beat him, let your friend summon you into his world, and repeat this process. After this time, you will both return home to Davis to reset the area. In short, 2 runs = 6 medals each, then reset. There are other maps that will give you an opportunity at killing 4 or 5 bosses each run, but they are much, much longer to run, and in theory, the bosses are more difficult. Happy grinding!